Advantages of diving multiple wetsuit layers
Wearing multiple separate wetsuit layers is a great cost-effective way to make one suit comfortable in a wider range of water temperatures. For example, you can make your One Piece Fullsuit work for colder conditions by wearing a Hooded Vest underneath.
Economical and Versatile
Adding a separate piece to a wetsuit is less expensive than buying another separate suit..it's like getting mulitiple wetsuits in one. You can dive the wetsuit alone in warmer conditions, and add the second layer when diving colder water or doing repetitive dives. If desired you can also carry multiple separates, like a 3mm and a 5mm hood.
Less to Carry
When traveling, space and weight are at a premium, so rather than taking multiple wetsuits on a trip, just bring an additional piece like a hood or vest. The smaller piece will take up less space and weigh less, especially if it's still wet.