Both. Most RO membranes are designed to operate efficiently at pressures of 50 to 80 psi. If your incoming pressure is lower than that you may still be able to make water but at a reduced rate and with a higher waste production. With a booster pump it normally gets you up close to the 80 psi mark so you get better production and with a lower waste flow.
I recently moved from city water to well water and as a result, lost a lot of water pressure and hence RO production. I got a booster pump and I'm wondering how the heck I ever lived without it. Man, this is one of those pieces of equipment you wonder how you ever lived without. I guess if you have a smaller tank it's not a huge deal.
A booster pump makes all the difference in the world if you have low water pressure or have a well. If you need one make sure to get the Aquatec 8800 for 50-120 GPD membranes. Many people end up with the wrong pump because of poor advise. FilmTec membranes also help with water production as they are rated for 50 psi while other brands need 60 psi or higher.