Why do later waves in siege have a timer?

I was playing normal difficulty in siege and reached wave 20 with 2 randoms, but they left, so I "tried" to reach 25 by myself.

However I didn't know it had a timer. I had 3 mins to fight 3 terminus units and waves of enemies with a dreadnought and 2 bots.

I would have completed the wave 25 if it wasn't for the timer.

Why did they add this? I haven't heard of wave fighting games having timers if you don't complete it in a certain amount of time.