There are many types of thermometers you can use, and you may eventually want more than one for task specific usage.
I use two, a glass dairy thermometer which floats, and a small dial thermometer with a six inch probe.
The important uses of a thermometer are to;
Make sure your wort or must temperatures are cool enough to pitch the yeast.
Make temperature corrections for hydrometer readings.
Monitor fermentation temperatures.
Monitor mash temperatures.
Other uses exist, go ahead and get one. Record the temps. in your logbook.