Okay, two major factors that come from not only full hopping but the fact that forward air is hitting in it's later frames from full hopping than short hopping (usually).
1. Safety
Used properly full hop retreating forward air is a lot safer on block. In terms of "distance" from your opponent after shield stun, there's a noticeable difference. This difference shows in that a lot of character's with powerful dash attacks will punish sh retreating fair on power shield very easily (Snake, Falco, Diddy, Zero Suit, etc etc) but have no/little out of shield options from the height Marth reaches. Otherwise, from a short hop, Marth is significantly more telegraphed in terms of where he will land, and also what he will do after landing. During a full hop ascension Marth reaches his maximum aerial mobility as well, giving him a stronger retreat. While Marth is floaty he's better than 2/3rds of the cast in terms of fast fall speed, and fast falling does have a slight impact on aerial mobility as well. The extra few frames gained from hitting someone on the later frames (like 7 instead of 4) has a slight impact in safety, also throws off people's power shielding timing. When fair's lag is over you're at around the apex of your jump at maximum aerial mobility and you can fast fall and be on the ground in not -that- much longer than if you did a rising sh fair into a ff, but you get a lot more "time" to space your landings with neutral air or fair. Characters like Olimar for example can power shield any aerial at sh range and usmash, dash attack or dash usmash, but he can not do the same from a full hop power shield. In general, Marth's best moves are aerials and one of the weaknesses abused over these years at many levels of play is ****ing him up for jumping.
2. Follow up
Full hop fair that hits someone chains into more forward airs at slightly higher percent, hence getting you more guaranteed/near guaranteed hits due to people tendency to DI upwards. Normally if we hit someone with a sh rising fair we would need to jump again for our next forward air to be horizontally aligned, an on hit full hop fair puts you at a much better height for a second fair, leaves you your second jump (for another follow up after the second hit).
Try watching Mike against RichBrown or Olimar's in general. Full Hopping in that MU is pretty much key to not getting ****ed up the ***.