If someone in my household has a Steam account and I create one also, would I be required to buy the games they already have in order to play them? Or could I play the games directly by finding where the files are stored?
Most games require Steam to work. If a game is single player, you can log in the other account, go offline and play with no issues, and the other person is not blocked, so as many people as you want can play the same game that way.
There is nothing wrong in doing so either.
It gets more limited for multiplayer games, however. But if they have LAN or some such, you can both go offline and play. However, you can't both be connected to the Steam account simultaneously as far as I know (it used to be possible, but I get disconnected now). If a game relies on absolutely no online account of any kind (even fewer games do that... I can think of.. Terraria?), then you can play multiplayer together with one account.