I use Service Types a lot.
Say I'm scanning a large countywide trunked system. I normally would scan everything but I often don't want to hear Public Works. I disable the Public Works Service Type and instantly all Public Works type channels are no longer scanner and I did not need to go in and avoid all the departments or municipalities that have public works departments. This makes it very easy and fast to scan everything but public works.
Same thing goes for scanning a specific service type like say the Fire service types. When something is going on involving fire, I can enable just the Fire service types and leave the others off and all I'll hear is fire related traffic.
Of course the same thing can be achieved using Quick Keys as well for different departments if you grouped like services under specific departments.
If you have fire talkgroups listed in several different departments though, selecting just the fire service types would probably be quicker than figuring out all the departments that have a fire service type in a given department and turning those departments on or off with quick keys.
Service Types are another way to make searching and scanning your groups or departments easier.
And no, you don't need to use them if you don't wish! It's really just a convenience feature.
As
@phask pointed out, new objects created manually in Sentinel or ProScan will be assigned a Service Type of Custom 1 by Default.
You just need to enable the Custom 1 Service Type and anything you manually add will be scanned. You could change every item in the scanner to Custom 1 if you wanted. That would make it scan everything.
As you mentioned, you can also use Sentinel's Profile Editor to enable any of the service types regardless if they exist in any of your favorite lists or not. This makes it easy to make sure new items you add with a specific service type other than Custom 1 will be scanned as soon as you write them to the scanner.
I myself still prefer to use service types though.