For me I was looking at a game I could repeatedly come back to and make some progression, then go play another game but when I had that urge for a game with longer progression I'd come back to Warframe. This has worked well for me over the last couple of years.

There's just something about the fluidity of movement and the deep customization of every weapon, frame, and companion. Plus who doesn't look some fashion frame? It's the real endgame.

All joking aside you don't need to buy platinum to succeed in this game, everything is obtainable and platinum can be earned by trading with players for rare mods and blueprints. Nonetheless if you get a 50% or a 75% discount definitely consider buying some platinum, you're supporting the game and the incredible development team whilst also allowing you to save time in some of the more grindy parts of the game.

I do warn you though and do not take this lightly, Warframe is one of the most grindy games I have ever played. If you don't like to grind then this game will burn you out, so try to enjoy it in small bursts between other games.