There are two ways to answer this. There's the trust Google answer and be skeptical answer.
For the trust google answer, they collect data to both improve their product offerings and to offer personalized ads. You can disable personalization in your Google account by toggling many settings, but it's unclear if they stop collecting the data or they're just not showing you the results of their data collection anymore.
Most of their ad revenue comes from specific services such as Search and Youtube as well as AdSense on third-party websites.
Many Google products aren't designed to grab your data, but rather to keep you in a comfortable and reliable ecosystem that they have control over or to sell you other services/products directly - such as Android,
Play store commissions, Google hardware, and Google One. Similarly to Microsoft, they want people to feel very comfortable using their products in their personal life...so comfortable that their employers choose to switch to Google Workspace just like how Microsoft gives out Microsoft office to university students, for free on the web, and sort of doesn't care about consumer Microsoft Office piracy because their main goal is keeping companies dependent on the microsoft apps to appease their employees.
Google Cloud services are also another big money maker, but I'm skipping it since it's not directly related to the consumer data harvesting.
All in all, if you trust Google, the best way to put it is that they want to monopolize your data, for better or worse. The cons are that obviously your data is being harvested. The pros though is that Google runs a very tight ship as far as security goes and they do a lot to protect your data from others.
The skeptical side is that Google isn't telling the whole story about what they're doing with your data. From a privacy and security standpoint, the best way to protect user data is to not collect it at all. Google still hands this data over to some outside parties like law enforcement upon request, so there's not much stopping them from handing it over to others as well if they wanted to. The only thing stopping them is their own privacy policy that is largely unenforceable and which they can also change at a moment's notice after already taking in all your personal data.
I will say that Google does get targeted a lot by privacy people, potentially too much, despite their privacy and security practices being far from the worse. It's really just the amount of data they have that is concerning, not necessarily what they've done with it. For most people, I don't think it makes sense to completely "de-Google," especially if you're just replacing google services with services that have even worse privacy or security. The more efficient solution would be to just limit how much data you give them instead of trying to eliminate them.
There are plenty of way worse services that are more deserving of being fully eliminated such as data brokers.