I checked the GBA board and didn't see a thread for these 2 games. Description of the game can be found in this link
http://en.wikipedia.org/wiki/Super_Robot_Taisen:_Original_Generation.
Reasons for TASing:
Extremely luck manipulation. Attacks can be forced to miss/hit as long as you are not facing 0%100% probabilities. Even better, the game shows these percentages before each attack. Also, characters with Luck attribute can even dodge/hit impossible attacks.
Attack animations can be turned off, making the game way faster.
Some missions can be ended "prematurely" by destroying a certain enemy, usually a boss. That is, when boss is destroyed, all other enemy units will retreat, thus ending the scenario.
Most text can be fast-forwarded with autofire/frame advance.
Not for TASing:
Both of these games are quite long, with 40 missions for the first game and 42 missions for the second. With some basic luck manipulation, I finished the 1st game within 297 turns and the 2nd within 403 turns. If not trying to get the "secret" last mission, these numbers will probably drop to something like 200 and 250.
Not destroying all enemies will lead you to have sub-par pilots and robots. This can make the game somewhat more difficult.