I second this question. I have heard a lot of strange things about Japanese workplace culture, but regular drinking parties with collegues are among the things I see most difficult to manage.

This is from my experience at a few companies and also a public university.

Here're some of the reasons for drinking parties after work:

1. Celebrate finishing a project
2. Celebrate the joining of a new employee
3. Say goodbye to a colleague who is leaving
4. Year end party, called "Bounenkai", for forgetting troubles of the past year.
5. New year party, called "Shinnenkai", in case you did not manage to have a good bounenkai
6. Accompanying a customer who traveled from far to visit you
7. If there was no other party for about three weeks, someone will organize one :-p

For universities and research labs, there are additional reasons like:

1. Getting funding from somewhere to do research (but we have to spend own money for thedrinking party)
2. Graduation of students, interns
3. Graduation of students or interns

The parties usually take 3500-5000 yen. For bounenkai and shinnenkai, the bosses will pay more than the others.

These parties normally take only two-three hours. But what happens after that, is, some colleagues will want to go for "Nijikai", as smaller groups. These will be more casual and fun, and they can select food/drinks as the groups prefer. Nijikai can drag pretty long and the last trains of all lines have a big share of nijikai participants. I occasionally see that some have thrown up on platforms etc. :-p

At the workplace, these parties create an environment where colleagues can put aside all the polite conversations and talk casually. Nijikais are perfect places to bash or badmouth others who did not join. So, the parties do serve as stress relief measures.

Some parties do feel obligatory. Then there are foreign workers who are non-drinkers, and are unhappy that they pay the same high price as others who drink alcohol do.

These parties do not have a good effect on the family. Most wives are unhappy about nijikais, even when they understand the need to go for some of the parties. Many surveys have shown that daughters in secondary school or higher dislike dads due to the dads keeping away from home with parties. Typically, Japanese housewives run the monthly budget and the husband has an allowance for parties, drinks, etc. Some workers have trouble when the allowance runs short. With declining average salary, some workers are finding parties troublesome to attend. One of my colleagues says he will have to finish dinner even if he is very full or very drunk, if he went to a party without enough notice. For university students, most of them spend money earned by working part-time, when it comes to the parties. One party costs a few hours of part-time work, so they are more reluctant. Most places have a discount for student interns, to reduce the burden on them. For me, I do not attend all parties that I am invited; that way, both the budget and time at home are managed. This is fine for a foreigner to do :o).