The first thing to realise when thinking about Tyranid tactics is that Tyranids only work when together, split up they crumble. This is emphasised by Synapse which forces you to keep your army together and punishes you harshly if you don't. I have found that if I attempt to spread my forces out then the Synapse creatures will be spread out too which can sometimes leave only 1 or 2 Synapse creatures babysitting 3 - 4 instinctive units. If your enemy targets the Synapse creatures and kills them then a large chunk of your army just went out of your control. If you are spread out then your other Synapse creatures will be unable to reinforce the de-synapsed side quickly enough for it to matter. If you group up then no matter which of your Synapse creatures goes down there will be another to control the horde.
The addition of area of effect (AoE) bonuses in the 5th edition codex has made sticking together even more important as it can give your entire army amazing bonuses and abilities which can often mean the difference between winning and losing. Venomthropes are an excellent example which make everything more difficult to kill but there are many other units such as Tervigons and Hive Tyrants which also provide many bonuses. When stacked these bonuses can turn even simple Gaunts into death machines; this is the whole point behind my Tyranid swarm list.
Another reason to stick together is that many of the most powerful Tyranids are slow and if split apart from weaker elements will not be able to provide any support, either in the form of AoE buffs or combat prowess. Tyranid units mostly work best when used in unison with one another with the best example being a Hive Tyrant and Hormagaunts vs a Tactical squad with a Powerfist. The Hormagaunts can tie up the powerfist and get Preferred Enemy while the Hive Tyrant can tear apart the rest of the unit with impunity.
Furthermore, most Tyranid weapons are mostly short-ranged so if the army is spread apart then your army cannot focus on one enemy unit or section. This goes against on of the most important rule of 40K, fight 50% of their army with 100% of yours.
Lastly the disadvantages. Sticking close together means you become more susceptible to template weapons which is why I hate my friends Imperial Guard so much. If you are not careful then those templates will destroy your army very quickly.
Additionally, sticking together means things get in the way of other things very easily. This happens a lot if you are trying to run forward ASAP but usually ends with incredibly bunched up models being vaporised by templates. The best way to avoid this is to understand some things need to be held back while other sprint forward as a first 'wave'. However if these waves are too small then they will blown apart as the second wave comes into range.
Another thing about 'over-crowding' is that if the enemy charges into one of your weaker units, say Terminators charging Gaunts, then it is usually difficult for your more suited units to get to the fight and help out because your own army is in the way. This is even more-so if the charged unit is large and so wraps completely around the enemy within 1 turn of combat. The best way to stop this is to have a fast reaction combat unit which can fly in the centre of your army. When something looks set to charge, fly your (Shrikes in my case) over behind the soon-to-be-charged unit and wait for them to absorb the blow before counter-charging.
Hope this helps all the other Tyranid Hive Fleets out there.