You might be amazed to know how critical Services are to functioning Apple devices. While they mostly can run offline, there are dozens and dozens of services that Apple runs that modern ecosystems require (like certificate related stuff). Other oddball things related to iCloud, APNS and the private services like iCloud relay are all extremely critical to billions of devices. Thankfully the all mostly fail open (captive portal is particularly tricky). Not saying they are as critical or visible as, say, Google.com going down, but none the less would have a very very large and visible problem if they all did go down suddenly. Thankfully, due to Apple design philosophy, most are totally decentralized and teams are given almost complete autonomy on how services are ran, which makes them a huge confusing mess but also, kind of a feature as Apple generally expects them all to fail in odd ways and the software can generally handle it.

But it is mostly used for periodic backup/sync. So you aren't going to really notice unless it is a very long outage.