There are performance benefits to your server. If combined only a single http request is needed for each file. If not combined an http request is needed for each individual css and js file. This can slow down page loading as http requests for an be somewhat slow.
The number of js and css files you have will be determined by the number of sp plugins you are using. This only applies to sp stuff not other wp themes or plugins.
Additionally, IE versions before 10 have a limit of 32 css files that can be read per page load. So if you have more than that css files fro wp theme, wp plugins and SP stuff, you will not get needed css loaded.