Sorry, too short.
It can help improve shelf life, but if you're sanitation is tip-top it shouldn't be necessary. If you're talking about doing it for something like the guinness clone recipe (in which you intentionally sour a portion of beer, then boil/pastuerize it and add it back to the main batch), all you need to do is heat it in a pot on the stove.
If you want to clean up wort for starters or something like that, look up tindallization, which is basically repeated boiling, and is an old school method to sterilize.
If you want to stop fermentation so you can back sweeten, that's another story. You could try potassium metabisulfite if you want to inhibit additional fermentation.
The heating process can cause flavor changes by driving off more volatile aromatic compounds, or destroying more delicate flavor compounds. If you end up boiling, you can sometimes cause additional isomerization resulting in additional bitterness.
What do want to pastuerize?