Comets are mostly unpredictable, there are a few 1p/Halley, 55p/Temple-Tuttle that come by Earth on more or less regular periods, that generally run from 20 to 200 years most comets are what are known as Long Period comets that have orbits that run from 100,000 to millions of years. Most of them are unpredictable, we often only know about them a year or so in advance when someone catches them on a survey of the outer solar system.
The community keeps an eye out for them for several reasons, many people are just passionate about finding comets, but also more importantly there is a small but real chance that a comet or astroid could hit the Earth and do real damage. Comet Shumaker/Levy 9 hit Jupiter about 20 years ago and we think a 2-3m rock from space probably hits the Earth about every 10 years or so. Now depending on where that hit it could do a lot of damage, so we try to know about it in advance.