To make sure you know exactly which mineral you have found.
There are over 4000 different minerals known to science, with more discovered every year. The number of minerals you're likely to find in the field is much less, but it's still an incredibly high number. It's important to perform many different tests so you can be absolutely sure you know which mineral is in your hand.
For example, you might be looking for azurite, a mineral that is known to be bright blue. The trouble is, there are plenty of other bright blue minerals out there, including covellite, bornite and some varieties of fluorite. You need to look at other features, such as the lustre of the mineral, to prove that it really is azurite. Azurite has an earthy lustre; covellite and bornite appear metallic and fluorite is vitreous.
It's all too easy to get similar minerals mixed up, such as quartz and olivine, or hornblende and pyroxene. You should always perform as many tests as you need to narrow the options down to one mineral.