Lower TPI tyres have thicker cloth and the gaps between the threads are bigger. These take on more rubber as the tyre is made, making a heavier tyre but with sturdy, abrasion resistant sidewalls. A low TPI count is good for a downhill tyre.
Higher TPI tyres take less rubber to fill the gaps so give a lighter tyre with lower rolling resistance as a result of having more flexible (thinner) sidewalls. Also the smaller gaps between the threads make it harder for a thorn to penetrate the casing. For racing, touring, or any performance oriented tyre, a high TPI count is good.