In PvP, my guess is you'll want your Mons to have one STAB charge move and one counter-coverage move so they're not toothless in suboptimal matchups.
Outside of PvP, there's not as much use for a secondary charge move for a good deal of mono-type Mons. Mewtwo being an exception, as SB and FB combined make it one of the most well-rounded gym sweepers ever. Groudon with Earthquake and Solar Beam allows it to function as a top pick for ground and grass-type DPS, thanks to it's huge stats. For many, however, theres usually one charge move that shines over the others as the highest DPS option. There's no point in your Espeon learning Psychic if it already knows Future Sight or your Raikou learning Thunderbolt if it already knows Wild Charge.
For Mons with dual-typing, however, this will provide them with an opportunity to learn a another STAB charge move of their secondary-type. This will provide some of them with greater versatility, by allowing them to perform as specialists in more than one role.
