You will need a filter for evry piece of lens. Primarily to protect the exterior lens glass from dust and srcatches. Most commonly fitted are UV Haze filter or the L37 filter. Quite similar but attenuate different wavelenght of UV rays.
Other type of filters are use to enhance the color of the shots. Like the polarizer cut reflections, deepen blue color of sky. Intensifier is used to bring up the redness color in trees, flower and sunrise/sunset. Some are also used to correct the color of light at different time of the day. Some will say you can use the white balance setting of the camera. This is one way, but not all the time will it give the desirable result. Combine with correction filters may help.
Oh there are many reasons you might use other type of filters like the ND filter. I can go on .... but I think you better go to Camera shop and ask a copy of Hoya Filter booklet and it should give you a genaral idea about filter and their application.
The most important part is do what it take to give a good photograph. If filter can help use them. If artificial reflected light can help use it.