Recently acquired an Everlast PowerPro to TIG and Have a Miller MIG and both are light enough to put on a cart and be somewhat portable compared with my old monster TIG machine which took 3 men and a boy to move. This isn't for a welding shop, this is for my shop at home where I only go through a couple tanks of each type of gas a year.

Question is:
Why do we use 75/25 Argon/CO2 mix for MIG, (on steel), and straight Argon on TIG, (both steel and alum)?
(I realize other specialized gases are available and used, but typically that's what I use.)

In the interests of possibly only having one tank of gas on my cart;
Why not just use 100% Argon to MIG and TIG or use 75/25 Argon/CO2 if you are only going to TIG Steel?
Any problems or disadvantages either way?

I am guessing that there is some reason dealing with shielding, but in many years of welding don't ever recall seeing why we use what we do.