More often than not, I find myself as a DM overlooking who is holding the light source in a party. I tend to assume that as long as someone in the party has a proper light source that the source is being passed along as needed.
However, there are situations where a player holding a light source can function as a detriment or challenge. For example, a party encountering any type of golem in a dark area will need to manage their light source so they can see since the golems have darkvision and don't need the light to attack. This could create a problem when your thief/rogue character doesn't want the light because they can't sneak with it, the arcane may need both hands to cast, the fighters/healers/tanks want to have a shield and one-hand weapon to be best equipped to battle.
A simple torch has created a situation where the party has to determine who holds the torch and could potentially give the attacking monsters a surprise round because the light hasn't been given to someone that could alert of the impending attack. So if you want to throw your party into a situation where they have to figure out how to work well with someone essentially having an arm tied behind their back, or if you want your party to think outside of their packs, throwing a situation like this at them could be a good test for them.