Permanent Residence. Blue Card holders can obtain a Permanent Residence / Settlement Permit quicker as described here.
Family reunification. Easier to get visa for your Spouse, without German knowledge. In addition, they can also work in Germany.
Mobility. If you already have an EU Blue Card from a different country for atleast 18 months, you can enter Germany for work without a work visa. You must apply in Germany within a month.
Living abroad. You can stay abroad for 12 months without losing permission to stay in Germany or EU. This does not mean that you can work from abroad. Your employer must allow you to do that for legal & tax reasons.