At its most basic, a vape mod acts as the power source for a vape kit. They generate power that is converted into heat by the coil in your tank, which vaporizes the e-liquid. If you’re new to vaping, rather than buying a mod and tank separately you’re better off buying a vape kit which combines both elements and removes the issue of compatibility.
However, if you’ve already made the switch to vaping and you’ve found a tank you really like, there are several reasons as to why you’d want to purchase a mod separately. For example, certain mods may be more functional, offering you more options to customise performance. There’s also the issue of practicality: your existing mod may have a built-in battery and be coming to the end of its lifespan which means it will need to be replaced. It may also be the case that you want to upgrade your mod to one that features a larger battery capacity, in turn supporting higher wattage vaping or to just last longer. Finally, there’s the issue of aesthetics, it may seem a bit vain but certain mods and tanks go together better than others. Or you may just need a smaller mod for a more discreet set-up.