1] Scratch my creative urge in public

2] Give substance to thought

3] Codify and automate processes liberating myself and others from drudgery

4] Build something that expands my luck surface

5] Communicate my ideas

6] Establish a brand presence

7] Participate as a creator and builder in the most revolutionary technical/organizational advance of my lifetime

8] Create value for people

9] Multiplier impact on my other skills

10] Potential business upside

Some of these are certainly true for programming more generally as well - some are specific to web development

Because you find it interesting and you enjoy it. If that's not the case, find another hobby / career.