Android manages memory on it's own and it does so very well, it automatically kills off sleeping apps on it's own as you open/run more things that require occupied memory. Apps running in the background don't slow you down because the programs you are using kill them off as they need to.
Task managers can slow you down because you can close programs that were running in the background, not hurting anything or slowing anything down, and the next time you open that program it will have to completely reopen rather than just be pulled from memory all ready to go.
It's not that there isn't ever a reason to kill of a service but that is usually going to be a single problematic service rather than to free up memory. This can be done from setting>running services, like was mentioned above