The main advantage of a separate battery charger is as soon as you put a freshly charged pair of batteries in your mod you can charge the next set you intend to use so you can swap them out immediately so there is no waiting time if you have a spare set of batteries.
Some mods can be vaped while charging but you must have your mod physically connected to something via USB, normally via a short cable - which means you can't go far from the charging point for it to be of any use. Also, if you decide to vape while charging YOU are connected to the same short cable and USB ports are very fragile - if you accidentally pull on the USB connection it is easy to destroy it.
Most mainstream Chinese mods have a charging circuit in the board, but most budget mods only have a guarantee of three to six months, so the components used tend to be the cheapest available. If during testing the manufacturers find that the components they use are likely to last for a much longer period than the warranty they WILL look for cheaper replacements.
A decent battery charger has ONE job, unless you buy a bargain basement £3 no-name charger superior components will be used than those in a mainstream budget Chinese mod.
When vaping you are probably only firing it for 2 to 3 seconds every hit - maybe a couple of minutes every hour so it is unlikely to get hot, unless you vape at very high power settings. When charging batteries in a mod, current will be running through the board constantly and heat can build up inside the mod, normally dedicated battery charges are open so heat build up inside the batteries and/or charging circuit is not an issue - most mods totally enclose the batteries so you will probably be totally oblivious of the internal temperature of the batteries which isn't good. If they get hot they will suffer localised internal damage which will dramatically reduce their life expectancy - the same applies to the board of the mod.
Battery chargers aren't very exciting but are an essential item for any vaper who uses mods with external batteries IMO. I recommend you use yours whenever possible and only USB charge in emergencies when you don't have access to your charger - and don't USB charge overnight if you don't want to burn your house down.
