There are two main reasons why.
Do you need to replace windows before siding.
If you re hiring the same contractor to do both jobs and siding is the priority ask if he or she will give you free window capping if your windows need capped when the time comes to install your new windows.
If you need to replace a smaller bathroom or larger bay window the process of how to remove windows will involve the same steps.
Depending on the type of window you are replacing it may not be as crucial to change them before the siding is installed.
Tips when installing siding yourself.
However if your budget prevents you from doing so it makes the most sense to replace your windows before replacing your siding.
If you are installing vinyl replacement windows into the frames of the existing windows it may not matter at all which is done first since the existing frames remain intact and can be properly flashed and sealed as the vinyl.
Nevertheless you can install the siding first if you have already started on that.
If your siding is in bad shape but your windows can hold up a.
Save up until you can get both the windows and siding done at the same time.
If you re going to replace the old windows with new ones the best time to do so is right before you hang the new.
Capping and moisture barriers.
Even in the best case scenarios having to remove surrounding siding and then replace it in order to properly mount a moisture barrier around new windows adds an extra step to the job.
If you are installing a new construction window in an existing home keep in mind that these windows often have a nailing flange.
Sfgate has a web page for how to install them yourself but it is very informational on the installation of windows before siding and why you should go in that order.
However your measurements power tools and supplies may change.
Typically installers will add new windows before they add siding anyway even when you replace both in the same go so you ll be following the order professionals prefer.
If you have to prioritize these projects do the windows first.
If you need to have new windows and new siding installed tackling both projects at once is ideal.