There is an argument to be made that this isn’t even a legitimate question to ask. Even if the games were similar, what would be the harm in having more than one game in a similar style on the same platform? Ask this sort of question of virtually any pair of similar games, and it sounds transparently silly. “We already have a ton of Fire Emblem games. Why do we need Fire Emblem: Awakening?” “We already have the Diablo series. Why do we need Torchlight?” The answer is obvious: variety. For someone who enjoys the core experience of these games, even the relatively small differences between Diablo and Torchlight, or between Fire Emblem: Awakening and its predecessors, are sufficient justification for the new games’ existence.
But let’s set that aside for a moment and talk about Telepath Tactics and The Battle for Wesnoth specifically. I was actually confused when people first started asking what the difference was; it is only a slight exaggeration to say that this is like demanding to know the difference between Risk and Chess! These games are about as different as two games in the same subgenre can be.