No, you do not need it if you choose the WIFI + unlocked version of the iPads. However, there are some benefits of the cellular version.
1) The cellular version comes with a gps. The WIFI version DOES NOT have a gps. The can be beneficial to have for some people, how like to have location.
...A) The gps does work without the carrier
2) You can decide to get cellar on any plan (Verizon, sprint, at&t, etc) later if you choose too
...B) it uses a esim, (Electronic Sim) not physical
However, the iPad with cellular is more expensive than the just WIFI version.