One of the great things about the guitar is that with the help of a pedal it can have an incredibly wide range of tonal characteristics. A single multi effects pedal will give you hundreds, or even thousands of different combinations of effects, equalizations, and expressions. This can be a powerful tool for the guitar player that allows for much greater ability to craft a desired sound and create the desired effect with music.
A guitar multi effects pedal contains a lot of power, but it can be used to create some simple, tasteful additions to your tone that many players of lighter styles of music will enjoy. Jazz guitarists will want some reverb and maybe some delay. A multi effects pedal can give you small doses of both of these effects in one unit, where otherwise you would need two different pedals. A jazz guitar player who needs to carry all their gear to a gig by hand will appreciate the easy of mobility a single multi effects pedal affords them.
Besides the lighter effects, a multi effects pedal can provide the type of drastic tonal changes that rock players will want. Most of these pedals are capable of multiple types of guitar distortion. Rock players will be able to have their distortion with the other effects that these multi pedals offer. You could have distortion with some delay, which creates a nice effect for a lead solo line with some presence. You can also add in some flange with most pedals to create a sweeping distortion sound. All of this would take up a lot of space and require a huge pedal board if you wanted to use the individual pedals for each effect.
The only downside to having so many effects in one pedal is that sometimes it can be easy to go overboard. There are some guitarists that use so many effects that they become more of an electronic musician or DJ than a guitar player. You want to keep the instrument sounding like a guitar, and use effects to heighten and amplify the effect of your tone, not create an alien sounding noise machine.
This doesn't mean you should not feel free to add heavy amounts of distortion to your sound or make a new combination of effects that radically changes the sound of your guitar, but your instrument should still sound like a guitar behind all those additions.
Multi effects pedals are the cheapest and most space efficient way to obtain a large amount of tonal possibilities for your instrument, and this should be taken advantage of. Make sure that you use the effects that they offer to increase the musicality of your playing rather than overwhelm it.