It's only there to Help you. Like if you want to Know How much Exp you have done today, You could check that and get an average exp, Or If you want to Know how much exp you do an hour with a specific Skill, Etc.
If you left it open while training, for example, stunning, it would show how much xp you have just gained from casting one stun. If you had xp multiplier items on it would show in brackets how much bonus xp you gained.