For one thing, ammunition for one weapon can be converted into ammunition for another weapon, allowing for the easier switching of guns based on what might be needed at the time. Personally, I've always been a fan of energy weapons. I hate that because in lore the energy weapons are supposed to be massively superior to guns. The game makers were really fucking stupid in this regard when they nerfed the energy weapons so that guns have all the advantages no disadvantages at all making the use of energy weapons pointless exercise.

The guns have advantages over the energy weapons in Falout New Vegas but there is no disadvantage at all when compared to energy weapons. The guns on the other hand only consume one ammo per shot, have as high damage as an energy weapon has, have a lot more common ammo types, easier to repair because you can find guns more easily and have different ammo types for different enemies like armor piercing and hollow point. I mean many great weapons like the Gauss rifle which could kick ass in FO3 was severely weakened by making it consume five microfusion cells, plasma weapons consumed more ammo per shot than in FO3, harder to repair because of rarity,etc.

In FO3 the Energy Weapons were vastly superior but in Fallout New Vegas the Energy Weapons were made so much weaker than they were in Fallout 3.

Fallout new vegas plasma rifle