it's all about pixel density.
For example, if you own a 27 inch 1080p monitor, you'll have a ppi (pixels per inch) of 81. A 1440p 27 inch monitor has a ppi of around 110. That's a pretty noticeable sharpness jump.
But, if you have a 23 inch 1080 monitor and compare it with a 1440 27 inch, the difference in ppi won't be as much as even a 10 ppi increase, so you won't notice the sharpness difference as much.
The main thing you're going to notice at 1440P is the amount of extra room for stuff on the screen and the sharpness will take a back seat unless you're using 27 inch 1080P or larger vs something the same size.