1) Beer intended to be laid down for a long time in the bottle (6 months upwards) will continue to slowly ferment at room temperature - this causes bottle bombs. If you can't keep your bottles cool all year round, pasteurizing will prevent this fermentation leaving your beer intact. This will, however, stop the conditioning process, so it should only be used if you are happy with the condition and flavour of your beer.
2) It's a chemical free way to stabilize beer, wine and cider. This also allows you to bottle a wine, cider or even a beer towards the end of fermentation, allow a few days for carbonation to occur, then pasteurize to stop the fermentation short - natural, sweet and fizzy wine or cider. There are other ways, but this is easily possible with equipment you probably already have.
You need:
1x stockpot, bottle height
Hot water
Home brew bottled in glass bottles
Thermometer
Rubber gloves
- Fill your stockpot about 3/4 of the way with hot tap water and put it on the hob.
- Heat the water to 88c / 190f.
- When you hit 88c, remove the pot from the heat and submerge your bottles. I find it's okay to leave the necks sticking out. Be careful of any very full bottles, and never lower any bottles with hairline fractures into hot water. It's almost boiling so watch your fingers!
- Leave them for 10 minutes for pasteurizing to take effect.
- Remove the bottles then leave to cool. If you have more to do, reheat the water and repeat the process.
For a large batch it's a little time consuming, but for 1-2 gallons it will only take half an hour or so.
Any questions, ask in this topic, but im no scientist so someone else might help you!

Cheers!