I answer questions like this all the time from undergraduates at the university where I teach. In general, yes, you should include work experience in technical fields. This work experience shows those reading your file several things about you:
You are able to hold a job. You know how to show up on time and work well with others.
You are a self-starter, choosing to go to grad school because you really want to, rather than drifting aimlessly and going to grad school because some undergrad advisor suggested it.
You have the ability to teach yourself technical skills and carry out what you learned at a high level. This is HUGE for success at research.
The kinds of things you thought about in industry might inform your future math research. Even if you get a PhD in number theory, perhaps someday you'll be doing math research that underlies the considerations of web developers. For example, perhaps you learn some nifty thing in grad school and realize it can be applied to modeling internet traffic on the web graph. We as a community of mathematicians get better math when we let in folks whose lived experience helps them create better mathematical models for reality.
Not super relevant as you apply for a PhD, but:
- When applying for a professorship, prior industry experience is also a strength, because, in addition to the reasons listed above, it makes you a stronger advisor. Most undergraduates will not go on to become professors. Most professors never worked outside of academia. A professor who has can give very useful advice to students. Furthermore, perhaps your skills could help the university in some way, e.g., by consulting for the University Communications department (who runs the uni webpage). After all, service is part of the job of a professor.
Lastly, there are many good threads about this kind of question over on academia.SE:
https://academia.stackexchange.com/questions/10931/planning-for-grad-school
https://academia.stackexchange.com/questions/202481/how-do-us-math-phd-admissions-committees-view-students-who-come-from-industry
https://academia.stackexchange.com/questions/62178/is-a-short-cv-problematic
https://academia.stackexchange.com/questions/59607/is-it-a-bad-idea-to-say-in-my-statement-of-purpose-that-i-want-to-pursue-a-phd-i