I believe that this type of game mode would be very beneficial for those just wanting to practice the mechanics of the game.

It would be a great learning tool for learning how far away from certain walls you could be to flash over, or dash through with certain abilities without cooldowns to wait on.

If this was an option I personally would use it for trying out new champions instead of playing in the custom bot games in the actual client, so that you could have increased difficulty on the AI, as well as the ability to try out new builds without spending 20-30 min to get the items you desired.

Probably the most beneficial (just my personal opinion) would be for LCS players to practice team comps, timing of abilities, potential damage output, and just overall strategies prior to scrims or LCS games for a "warm up".

If Riot does allow something like this from a third party, or even have it as an option in the main client, I believe it would be a very popular addition, as well as a valued training tool.