Is there any benefit from merging sites?

Question

Search Engine Optimization

There is a client, a construction company, which makes a separate website for each of its new projects. And of course there is the main site of the UK itself.
And there is an idea of ​​combining all sites into one network, under one domain and different subdomains.
What are the pros and cons of this approach for SEO/SERPs, or whatever?
PS All projects are focused on +- the same region.

Answer 1

Is there any benefit from merging sites?

I do not see any pluses, except for the cost of domain maintenance. My advice, do not touch the old ones, do the new ones on subdomains.