Most updates are just for performance and bug fixes (unlike major upgrades) so not big features are normally in updates but you'll still have those changes for the collectors and server active even if the agents are not restarted. Only changes affecting the agents themselves won't be applied until they are restarted.
So if there is a collector bug that is fixed in an update that will go into effect after the collector is restarted regardless of whether the agents is restarted and updated.
Each update has a release notes attached that will detail what changes went in place and is broken down by components so you can see if the agents received significant changes. Any critical or high importance ones are highlighted at the top. These can be found for any update on the downloads page. Here is an example of one: