Not sure why this is being downvoted. It hits the nail on the head. If you are concerned about privacy around advertising then using a browser from the biggest online ad company is short sighted.

Quite a lot of reasons. I assume you asked that because you're thinking it's used to gather information on its users. That could be one of the many reasons. At least initially it was because Mozilla/Firefox didn't want to adopt a multi-process architecture.

In terms of strategic reasons, as a company that depends on people browsing on their websites other reasons are obvious: avoid lock in that could be pushed by third-party browser makers/competitors (say IE becomes the most popular and it implements proprietary extensions that work only on their websites[1]), ensure there exists a fast secure browser so that people can keep browsing even if everyone else stops making good browsers out there.

[1] Now before you go ahead and point out how Google proposes HTML/HTTP features that get implemented in their browsers and on the server side, all such features have public specification and source code, so anyone else could implement them too. This is very different from the IE days of yore, where MS was extending IE through ActiveX. ActiveX was developed in house and they were releasing binary plugins/SDKs to develop ActiveX plugins, effectively maintaining full control over it (one would have to develop ActiveX compatible technology from scratch if they wanted it open source, with Chrome all they have to do is fork the source code).

so that you don't have to pay royalties to other browsers for being the main search engine. I mean you have to pay one less. And if you have the most used browser, you save a lot.

When Chrome was first developed, browsers and the web were relatively slow, and slowing down due to the popularization of Javascript and heavier websites.

Google's worked on a number of technologies to make the web faster; Chrome (and V8), their own DNS, image and video compression technologies, AMP, HTTP/2 (SPDY), HTTP/3 (QUIC), webserver plugins (mod_pagespeed), benchmark tooling (Lighthouse), and extensive guides on website speed optimization.

The reason is simple; faster internet = faster browsing = more page views = more ad impressions + more behaviour tracking data points. And it's a win-win for Google as well, because it earns them goodwill (well, except for AMP); especially at the time Chrome was a breath of fresh air compared to Firefox, and it's taken a lot of time and effort just to keep up, with mixed results (to the point where a number of manufacturers have just given up and adopted Chrome's renderer).

A better question is to ask why people continue to let themselves be confounded by a browser made by an advertising company.

Google is a total-spectrum surveillance company. Advertising is a product they offer to their clients. (No, that is not you and me.)