I believe he wants a means of 'testing' any champion he chose before purchasing them.
Here's the issue, testing a champion in battle training doesn't actually give you a good feel for how that champion handles. Heck, even coop or your first few pvp games don't really give you a good feel for them. Beyond that, having 100+ champions to try to choose from is actually rather intimidating for most players. Try playing DotA and having any idea what's going on in your first 100 games, when people aren't mostly using the free week champions. (Here's a hint, you might know what's going on in 5-10 of those games.)