TRAITOR!!!1onehundredandeleven
Not like anyone reads this but Aegis in itself is a versatile super. The most practical and simplest of uses is for pressure control. If someone is rushing you down and you got meter, let out an aegis and stay near it. Sometimes people make mistakes like throw fireballs at it and you can follow that up with a tackle for some nice damage/pressure.
You can use it to lock someone in the corner. If they jump and get hit, they fly back into the corner and you can combo them till kingdom come with an appropriate/character specific tackle sequence. it's like a wall of fire that can't hurt you. Use that to your advantage. You can jump through it for some nice grabbing while the opposition wonders what you're planning on doing.
Fault is right about that juggling thing. There are times when you are in the corner and you get a juggle started and you may opt for some major damage. Doing a mk tackle xx hp aegis facilitates that nicely. You can charge during the aegis to follow up with about 3 more tackles if you can charge buffer well and also depending on the juggle limit. I don't really do this often though.
The last and bestest best use is for unblockables. Some of these are a lot too advanced to explain here. I may make short vids or do something as equally pointless on this.