The case is ownership
Search visibility and social reach are borrowed and can change without notice. A list of people who asked to hear from you is the only channel nobody can alter — which matters more as answers replace clicks.
The cost is a permanent commitment
A newsletter is a promise to keep producing something worth reading, indefinitely. Most stop after a few issues, which leaves a list that has forgotten you and reacts to a sudden reappearance as spam.
Don’t start a newsletter you can’t still be sending in a year. The abandoned list is worse than no list.
Don’t treat it as a distribution channel for your blog
“Here are our latest posts” is ignored. Something written for that audience — a specific observation, a lesson from a project — is what gets opened, and it can then point to the fuller piece on your site.
It is not your primary call to action
Subscribing feels productive and is a softer substitute for the thing you actually want. Keep it well away from the one action a page is asking for, or it will quietly cannibalise enquiries.
Deliverability is your problem now
Sending bulk mail from the domain your transactional mail uses risks one damaging the other. Keep them on separate sending identities — the same separation transactional email requires.
Judge it by replies, not opens
For a services business the signal that matters is people responding, mentioning it on calls, and enquiring. Open rates are increasingly unreliable and were never the point.