Apple Watch series 5 w/ cellular

Only if you want to. The Watch will still work without activating the cellular service the same when you have it hear the paired iPhone. If you wish to use it without the paired iPhone nearby, then you would need to activate cellular. Cellular providers change an additional fee on your account for the Apple Watch cellular. Here in the US, that is generally $10/month.

A Cellular Apple Watch Without a contract and the GPS only Apple Watch can do  Everything  the activated cellular model can do  as long as  your iPhone is  nearby, where nearby is somewhere in the same 3-4 bedroom home (assuming typical wood frame construction).

https://support.apple.com/en-us/HT205547

An activated Cellular Apple Watch allows you to leave your iPhone at home while you go for a walk, run, bike ride, et...

The activated Cellular Apple Watch shares the iPhone's phone number.  This requires the iPhone cellular carrier to support the Apple Watch.  The iPhone carrier charges a monthly fee for this service.  In the U.S. the fee is $10 per month added to your iPhone bill.

iPhone Cellular Carriers that support the Apple Watch, which models, the radio frequencies each model uses, and the country where they offer service:

https://www.apple.com/watch/cellular

You're welcome.