The save button is not useless. In Titan Quest, your game is saved whenever you find one of the fountains and save at it. But you can update your quest status and other such details in the current save by using the "Save" button as you play before you reach the next fountain.
So, for example, let's say you find a fountain, and it saves your progress. You might then continue playing for a while, find a couple of cool new items and maybe complete a quest, but you don't have time to find the next fountain before you quit. If you just quit, you'll lose those new items and the quest progress. If you save the game before you quit, your items and quest progress and what not will be saved.
Either way, the next time you load your game you'll find yourself at the last fountain at which you saved. But by using the save button you can hang onto items and quest progress.
In response to the comments, let me add the network details as well through an edit. If you're playing a network game of Titan Quest, you're only saving your own game. The host determines which quests are available in the first place, so if he's already completed stuff you haven't, you won't be able to do it.
But when the host saves his game, it saves only the status of quests, items, etc. for his character. When you use fountains, it saves quests, items, and respawn point for your character, and when you use the save button it saves quests and items for your character. So you should still use the save button when exiting a multi-player game as well.