1) DH is 1 of four and they are all 18 months apart, and he remembers how great it was because they were all close in age.
2) We wanted a big family and we didn't want to be 35-40 and still having kids.
3) Our childcare is an au pair, so our expenses are "fixed" no matter how many kids we have. It is beneficial to have them closer together, and when the last one is in preschool we will not need one anymore, and save on that as well.
4) Reusing clothes, if our kids were farther apart I don't know if I could convince myself to keep the stuff for 3-4 years (I already find it hard to hang onto the stuff from DS1 and DS2 for DS3 (there is a 4 year age gap between the second and third boy).
5) Diapers, if I was ever "done" with diapers I would never want to go back!
I could go on and on, but these are all personal reasons why it works for us, everyone is different, GL