Because it can!!!
There is even a website about it with a very convenient name: petsintheclassroom.org
See told you the name was convenient
Pets can do amazing things in the classroom and this site talks about so many of them. Pets teach responsibility, build self esteem, encourage nurturing and can even become friends to students. On top of all of that pets in the classroom can also be used in tons of school lessons like observation skills, and long-term studies. Because of this, I will have pets in my classroom and I hope you have pets in your classroom too. Check the literature, its positive!