When is better to swap top and bot lanes?

Solution 1:

There are a few reasons to swap lanes.

1. To get golems to the adc/support. Most of the time when you see the adc/sup go top its when they're starting purple(top right). This allows them to keep up cs/level with the opponent's bot lane adc/sup.
2. Sending the adc/sup to a 2v1 lane will most likely lead to an early tower kill if the lane opponent isn't good at wave clearing. This gives the team more jungle presence(blue golem) in the opponents jungle, and is also why adc/sup sometimes go mid. This works out even better if they can take out the early turret and get back to the lane with the opponent's adc/sup before they can get their turret making them a turret up.
3. If the teams adc/support won't trade well with the opponents adc/supp (range or sustain or whatever). This is so the adc/support don't get shut down and pushed back to turret. Competing with the turret for cs gets annoying :)
4. Finally, and less likely, if the top counters the teams current top champ hard. Sending a adc/sup against an opponents top will most likely shut them down cs wise and make it harder to get early farm/snowball the lane.

One thing to remember if you're going to send your adc/sup top is that if they cant get the tower down, and get back bottom lane by 4-5ish you're risking a dragon to the opponent (their adc/support is still bottom and able to help with dragon while yours is on the other side of the map).

Another important note, as of patch 4.1, top and mid turrets gained 20 damage reduction from champion basic attacks. This makes lane swapping even more risky as the duo that goes bottom won't have this damage reduction to deal with.

Solution 2:

Trading top with bot makes you give up dragon control so the duo lane (ADC and Support) must kill the tower fast and then go back to bot to make sure their tower remains standing.

It can be done with great pushing duo comps, such as Nunu with Caitlin.

However you make it harder for your solo laner to last hit. If your solo is playing a champion that can easily last hit from range such as Elise, it is possible to take the first tower with your duo, gank bot, and then take dragon.

Another important case where such a swap may occur is when the solo laner is counter picked.

In general this is high-risk, high-reward play.

TL;DR

To quickly take early global objectives (turrets and dragon) and to deny the enemy solo lane.