In my opinion a capo is an essential purchase for any beginner guitar player, and if possible I'd recommending buying one along with the guitar so you have access to its uses straight away.
It allows you to play lots of songs straight away!
The main reason why the capo is so useful is because it allows us to play 1000's of songs using the most common simple chord shapes, this helps a beginner get off to a great start as you'll be able to learn all these great songs whilst developing you ability to form chord shapes and increasing the speed in which you can transition between them which are two of the most important goals for any beginner learning guitar.
Without the capo you are more limited to the amount of songs you'll be able to play and may have to resort to playing the same songs repeatedly or using different exercises in order to develop your playing, the capo helps us keep things fresh and new as you progress.
Using a capo also makes playing chord easier!
When your a beginner, sometimes you may have trouble pressing down on the strings in order to find the desired sound, this is usually because you haven't developed enough strength in your fingers yet, or perhaps your fingers hurt due to not yet having built calluses on them.
Placing a capo further up the neck helps to bring the strings closure to the fretboard, therefore you won't need to apply as much pressure in order to get the sound you're after.