Quality is especially important for the cable from your instrument, as this is conducting the weakest signal. Speaker cables connect the amp to the speaker. They are not shielded and they use large diameter wire, typically 12 gauge. These cables must conduct a large amount of power to the speaker without losing information or altering the signal in any way. All wire is not created equal and all copper is not the same.
Acoustic instruments have many subtle nuances that can be lost by using inferior cables.
You don't have to use the most expensive esoteric cables but good, solid quality is necessary to get the best from your amplifier and your acoustic instrument. Never use a Y cable to sum two signals, this can damage your amp. Always be sure the amp is connected to a speaker and don't connect the speaker output of an amp to anything other than a speaker. I've had people do this, with disastrous results. By connecting the send and return jacks of the pre- amp to the return and send jacks of the effects unit with patch cables, we make a loop in the signal path, yep it's called an effects loop.