It's about the detail
Having a device to test and use in-situ will ensure it will work for you. Any weaknesses in the batteries, scanners, accessories, custom software and all the small differences in each device you are considering will be uncovered as you test.
Learning and project sponsorship
An evaluation device will get technology into the hands of your users early, aiding strong project support from your workers and project sponsors. Involving users early will uncover issues that could be costly if found too late.
Reducing risk
Actually seeing and working with an evaluation device will make sure it will be accepted by all areas of your proposed solution. You'll install technology that benefits, not hinders your users and business.