It's easy to come up with some random topic to write about. And it's also easy to start writing about it. But sometimes halfway through you either run out of ideas or notice you have diverted writing about something completely different.
Without a good plan and a topic that has enough subject matter, it might be hard to keep on the subject even for a few hundred words. An idea might feel like easy to write about, but does it carry all the way to the end.
And even if there would be enough to write about the topic to fill the page another issue is losing focus. Along the way, you might come up with another idea you want to write about. And if you are just writing in the flow, guided by your stream of thought those sidetracking ideas popping into your head will just end up in the text.
It's a shame actually. With a little more effort in sure those topics that run out of steam halfway through could as well be extended to a full post with a bit more effort. And when you start writing about something else it's a wasted topic. You could have had two days worth of things to write about instead of just one.