It would be a convenient option to have , as it will simplify the event logic to fire an action every X seconds when a condition is met.
Where it would be useful:
So for X in the seconds we can have a variable called
“enemy fire rate” > create bullet
or “deplete health when stepping on spikes” > reduce health
or “create footsteps rate” / player is moving > create footstep at player position
GD has “Timed events”, search them in the Extensions dialog 
Here is some info from the wiki: Timed events on the Wiki
To make a timed event fires its sub-events every X seconds, set X as the time in the timed events dialog, and finally reset the timed event from its name, something like this:
Note that the internal timer of the timed event is not increased if its parent event doesn’t run, and that you can reset the timed event from any other event
Its not available for html5 games 
also I wish this was a standard timer condition and not an extension. The way it is in construct2 and clickteam fusion.
It would also be more consistent with the rest of gdevelop. The timer should be in the conditions (left) column. That would be much more straightforward. Currently it looks like a workaround way , not very good , not as straightforward and clean
There are also timer’s conditions available without the extension.
but it’s not as clean as the other engines - in a single block of condition logic. 
Having a single condition block doing that can greatly simplify your events. It’s a common event condition in game design, so I dont understand why it is not available as a single block and must be done manually with a bunch of them or via an extra addon that doesnt even fit with the editor design (its not a condition) and is not available in html5.
Maybe not that important of a feature, but sure does save time to do it in a block.
Yep, do every X seconds is the only thing I miss from MMF. I know there are other ways to do it, but the way it is done now is very clumsy and not very intuitive for new devs.
if its easy to implement - why not? 
Look how nice and simple it is when you have it:
it can be used in so many ways:
to make an action repeat less than every frame and save some resources for example (i do that in mmf sometimes)
The more convenient and intuitive you make workflow, the more people will love the engine.
yes, there are other features that need to be implemented and are way more important atm. This is a convenience feature.
Just as a “pin” behavior would be.