First of all, even though the independence debate is very important, the current stage of the "conflict" is whether Catalans have the right to, not simply decide about this, but to voice their opinion (in a non-binding way) on this.

This said, I will try to explain some of the arguments and thoughts that are common amongst many Catalans supporting independence. Obviously, not all share the same thoughts.

Catalans know they are very important in the Spanish economy. The BBC article has some graphs and explanations on this. Catalans also know, and this is public information, that a lot of the money they contribute in taxes never comes back and is instead used in other regions. This is accepted, but there's a perception that the money is not being used to boost a productive economy in other areas but to subsidize non-productive sectors as well as to finance services and infrastructures that are not critical or necessary, while many of those services and infrastructures are not available in Catalonia. Some nation-wide (here nation means the current Kingdom of Spain) infrastructures have been designed with a centralizing vision of Spain that doesn't consider certain objective variables. One very well known example of this are the high speed AVE train railways, which are unprofitable and the EU criticized [1]. On top of that, Catalans see that this has been the case for decades and, if anything, it looks like it's getting worse. [2]
The Catalan proposal to some of these problems is to make sure that, when the money is redistributed amongst the Spanish regions, Catalonia (or other regions) doesn't lose it's position in terms of GDP. For example, if Catalonia is the 4th region in GDP in a particular year before redistributing taxes, it should still be the 4th after the redistribution. This was included in the 2006 Statute of Autonomy of Catalonia, which, as I'll explain now, the Spanish Constitutional Court ruled unconstitutional.
The concern is not so much about the money that could be kept in Catalonia, but about how it could be used for services and infrastructures that are necessary, productive and that boost the economy.

That's where the taking advantage part ends. The other arguments are not so much about this but about how Catalonia cannot make and execute important decisions about education, culture, economy, infrastructures, services and relationship with other nations that would often follow a model very different to the Spanish one.

To mention but a few:

* The Statute of Autonomy of 2006 was the main law for self-government that the Catalan Parliament passed and that was approved in referendum. This law was the result of years of discussion and hard work to build consensus. It was ruled unconstitutional in 2010 after years of deliberation. Similar situations occurred in the past. Please take a look at picture [5] (demonstration after the 1932 Statute was rejected) and at the Wikipedia article [6] about the 2010 demonstration after the 2006 Statute was rejected..

* The model of education in Catalonia is a case of success and social consensus. Education is mainly in Catalan although Spanish and English are thought as well. This is one of it's main features but the Catalan model differs from the Spanish one in many other ways. Catalans regard this model as the main reason why the Catalan society is a cohesive one, even though Catalonia has received huge amount of immigrants continuously during the last 60+ years. During the last few years Catalans have seen how the Spanish Government is trying to change this model, including the language.

* Catalonia is not recognized as a nation. The Statute of Autonomy of 2006 included a mention of Catalonia as a nation. Many Catalans feel this as a violation of their identity and right of nationality. In consequence, the Catalan culture is often silenced and ignored in all international spheres.

* Spain has repeatedly banned the adoption of Catalan as one of the official languages in the EU, even though more people speak it than many other official ones. [2]

* Catalans cannot participate in international events as Catalans. Take sports, for example, where Catalans have repeatedly showed that they would support them participating internationally [3].

As you can see, independence would simply allow Catalans to decide about all these things. Many Catalans felt that the best way to accomplish these goals was to negotiate patiently, but years and years of unsuccessful results have convinced them that we no one should decide for us in our internal matters as well as relationships with other nations.

An independent Catalonia would probably do very well by itself, according to many. Mid and long term it's all benefits, even thought at the beginning it would be difficult given the current condition of the Catalan finances. The biggest risk is if Catalonia ends up outside of the European Union (which Catalans don't want and it's unclear that it could legally happen) and the consequences of this. You could take a look at [4] for more details.

For many Catalans remaining in Spain is simply not an option anymore.

[1] http://www.lne.es/asturias/2010/12/14/ue-critica-modelo-radi...

[2] http://www.vilaweb.cat/noticia/4072536/20130109/catala-seten...

[3] http://www.324.cat/noticia/47238/politica/Pujol-recorda-que-...

[4] http://www.wilson.cat/en/comunicats-conjunts/item/215-dos-m%...

[5] http://lh6.ggpht.com/_KKTCT60suEU/TXU5tANkPEI/AAAAAAAADHk/La...

[6] http://en.wikipedia.org/wiki/2010_Catalan_autonomy_protest