The WiFi+LTE iPad Pro has both an embedded SIM (eSIM) and a regular Nano-SIM card slot. Useful if you need cellular connectivity on the iPad frequently or if you'd like to use different carriers for better coverage. For example, I use AT&T on the iPhone and Verizon on the iPad. My old iPad Air, I put on a promotional T-Mobile 5GB/150 days for $10 prepaid data plan.
If you want to use the built-in cellular, the iPad will need its own line of service (could be postpaid or prepaid). Signing up for a prepaid data plan on the iPad is very easy in the US. You can view and sign up for plans directly on the iPad (Settings - Cellular). Postpaid, it's usually $10-20 per month to add the iPad to an existing plan.
If you're traveling internationally, you can sign up for data plans in select countries and carriers with the eSIM (or Apple SIM for Air 2, 5th gen and 6th gen). In case the country you're visiting isn't supported by the eSIM or Apple SIM, you can always use regular carrier-provided Nano-SIMs as fallback.
I like them because they're convenient. Having an always on data connection is also extremely useful if you carry the iPad around with you. My dad has a WiFi-only iPad and he misplaced it in the mall one time. We were lucky we were able to get it back after an hour of searching (no internet so can't use Find My iPad). Probably would've taken all of 5 minutes if his iPad had built-in LTE. He's been banned from taking the iPad out of the house now.
TL;DR
Built-in LTE is a convenience. Whether it's worth $130 plus recurring data plan costs is up to you to determine.