Tenses are simply the time in you choose to roleplay.
It depends on the RP. When does the RP happen?
Different tenses express the information you want to say in different ways. For example, some RPs are better expressed in the past tense. I, TheColdHell, personally prefer to RP in the past tense. Most other people prefer Present.
The characters all have to be in the same tense. If you have two characters, for example, they both have to be in the present tense, not one in past and the other in present.