You don't need in the sense that you need water in order to survive but unless you want to lose VHF stations then I recommend buying an antenna that receives both.
This fact, of course, only applies to you if you receive UHF and VHF in your area. It also doesn't apply to indoor antenna seekers because certified indoor antennas are capable of receiving UHF and VHF.
Determine If You Have UHF and VHF channels In Your Area
Antenna Web is a great website to get a list of TV stations in your area. Their Choose an Antenna tool is an easy way to get your channel list; you'll also get an antenna-type recommendation that is customized to your street address.
I thought this point needed to be addressed when buying an antenna because the majority of outdoor antennas are UHF-only.
So, if you want an outdoor antenna capable of receiving UHF and VHF channels then be sure to read the antenna's specifications before you buy it and verify that it receives both.