It’s generally not a good idea for a web designer to put a “designed by” link in the footer of websites that they create, and they usually know it too.
The main reason is Google; putting that link down there will hurt the site that it’s on. It’s a tiny amount, but it’ll still hurt. If you’ve developed a site to help a company, why would you do literally anything that actively hurts it?
In theory it helps the designer rank a little better, at the expense of the site they’ve built, though it could end up hurting both parties. As search expert Cyrus Shepard has said:
A couple of other rules that I see people violate all the time that Google has made painfully clear in the past few months: Don’t link externally in the footer. Just don’t. I’m not going to go into the reasons. Just don’t do that.
Why would they do it?
If all of that is true, then why do so many people put their links down there? I see two possible reasons.
They don’t know any better
They see other sites have that link, and just think they’ll follow suit. While a pure designer can perhaps get away with that, anyone with any degree of overall web building skills should know better. If this is what you do for a living, this is 101-level stuff.
They do know better
Even worse, some know exactly what they’re doing. I’ve gotta think this is way worse. They know the link will help them and it’ll hurt you, but the pain is so small that you’ll never notice. After they do this to 10 or 20 sites they’ve built, it could help their rankings go up a little bit.
They’re both awful reasons, and there is no good excuse for doing it. If you see someone putting those links in the bottom of sites that they build, ask them about their reasons for doing so and see where the conversation takes you.