In some games, the activity is complex and interesting enough to be repeated. That's fine. However, most games should ramp up.
If a game doesn't have a ramp, it should probably be a short game. Many of my games are rampless — they just get straight to the action, stay there for a short while, and then end. However, I'd generally recommend having your game ramp up. It's a tried-and-tested approach, and is much easier to execute than a non-ramping game.
Scrabble does not have a ramp. In Scrabble, players just repeat the same activity, until the game ends.
In Magic, players play a land card on each of their turns. As the players have more and more lands, they can play bigger and bigger spells.
Players like going up ramps — building or strengthening or adding to their presence in the game. It's fun and creative, and ties into people's constructive instincts. Players feel like they're making progress.
A ramp progresses the game to its end. More importantly, it keeps the game changing, and it keeps outdoing itself, with bigger and cooler stuff.
It might be tempting to think "this is just going to snowball, and one player will get out of control." Yes, but that's the fun. Seeing whose game can snowball the fastest. Don't make it too simple, of course.
A proper game should have multiple ramps. If there's only one ramp, it's obvious who's going to win, and it's too simple. Give each resource or system its own ramp.
Your ramp should be steep. The bigger the ramp, the better, within reason. Your game should begin with the smallest possible actions, and eventually reach actions that almost break the game. Then, it should end.
At the beginning of the game, a player is weak, and weighed down with restrictions. Through the game, the player gains strength, and overcomes these restrictions. This isn't just good for the game. It's fun and rewarding.
Avoid having your ramp be reversible. It's okay that some things are reversible (such as resources, money, health etc.), but at least one important thing should just go constantly upward.