Enzymes are affected by temperature. The closer they get to their de-nature temperature, the faster they work. Beta amylase enzyme denatures @149-150F. Alpha amylase denatures @165F or so.
In the real world, it would be best to have those two flipped around, because alpha needs to work before beta can finish the job. Alpha splits the starch molecule down the center and creates lots of "ends". Beta can only work on the "ends" of the molecule, that's why the the alpha needs to go first. So how long does it take to convert depends on temperature. The lower temperatures and pH favors the beta enzyme, but the alpha is also working, but at a slower rate. When you get above 150F, the beta enzymes work the fastest, but are quickly denatured. Alpha continues to work up 168-170F. At those temperatures the alpha enzyme works the fastest, but like beta is quickly denatured.
Beta creates the most fermentable wort, so lower temperature keeps the beta from going away. Beta converts the starches into mostly maltose, sucrose and glucose. Alpha works at the lower temperatures, but again, at a slower rate. Alpha converts starches into dextrin, but it can also produce fermentable sugars like maltose.
Mashes can convert fairly quickly, but the problem lies with the enzymes could still be working on creating simpler sugars or more fermentable wort. So stopping the mash short reduces conversion efficiency and fermentability.
145F mash takes more time 60-90 minutes depending on the beer style.
152F works much faster, but the beta enzyme doesn't last very long, thus less fermentable wort.
158F the mash converts very quickly, but contains more dextrin because the beta didn't have much of a chance to work.
168F is super fast, but alpha is soon disable or denatured.
So why do a mashout? To get the most out of the mash as you can and to a lesser extent, denature amylase enzymes. Mashouts are not necessary. But like I said before, if you can, why not?
Sorry to be so long winded.