With a time word, to talk about an action which starts before a time in the future and continues up to that time.
In April 2009, I will have been teaching here for two years. (=I started in April 2007 and still teach here now, probably I will continue after April 2009 but we are not sure)
We often use this tense (instead of the present perfect continuous) because we like easy numbers.
For example, imagine now it is March 2009. I started working in my job in April 2007. If you ask me: how long have you been working here?’, I don’t want to say ‘1 year and 11 months’ because it’s a bit long and complicated. I prefer to use the future perfect continuous so I can say 2 years, which is an easier number.
So, instead of saying: I’ve been working here for 1 year and 11 months (using the present perfect continuous)
I prefer: In April, I will have been working here for 2 years.