Do you mean a Bumbo chair?
I have one and I would advise you to buy one that includes the tray. The tray is difficult to put on but it comes in handy when your baby becomes more mobile and tries to "escape" the chair. The tray can hold my baby in when he struggles in it.
It is not advisable to use the chair on elevated surfaces in case of falls. So I usually put the bumbo on the floor.
If you're considering putting the bumbo on a dining chair, you might want to consider investing in a booster seat instead.