You still can have access to the iPhoto Database, where all photos that you decide to store through iPhoto are kept.
iPhoto gives you (somewhat limited) editing and organization of your photos.
And some other features that you may or may not use, such as Faces, which allows you to label a few examples of faces, then iPhoto would suggest sometimes-accurate names for people, by interpreting facial characteristics. Sometimes useful, sometimes not so much.
Used to provide albums that you could order over internet, using pictures that you chose from your own collection. That's not available anymore, as far as I know.
Maybe bottom line is that the photo collection through iPhoto/Photos, nothing particularly special, unless you are OK with what you get. And other methods/software, whatever is your choice, is good, too!
Apple never has required that anyone use iPhoto (or the Photos app, which replaced iPhoto sometime during Yosemite, IIRC. )
But, iPhoto/Photos stores all the pictures in a single storage file, which limits your open access to your pictures, unless you understand the simple tips that are necessary to get at your pictures, whenever you choose to use your pictures in another way. Some folks think that Apple makes that process too challenging.
But, the fact remains that you still can get at the original photos, as you originally saved them, if you want to jump through a couple of hoops to do it.
Advantages? New Macs come with the software, making it useful without having to use other software.
Does it serve everyone's purposes? Of course not.
Does it do a reasonably good job, considering its limitations? Conditionally, yes.
If you don't like iPhoto/Photos, then you can choose to ignore it. It is not "imbedded" in the system like iTunes is.
