The beginner keeps re-inventing the wheel with every drawing.
The pro takes some extra time to create a refined wheel that doesn't need re-inventing.

...and by "wheel" I of course am talking about all the designs used in a manga: characters, scenes, clothes, etc.

Experimenting with your character designs and other things is very important, but you should do it seperately from your actual manga pages.

For an optimal reading experience, it's vital that all your designs stay consistent across all panels and pages. Even if you find a much better way to draw a particular thing, you can't just change it midway. Instead, evolve it slowly across many pages.

A common excuse many beginners use to justify their drawings is: "That's my style!" That's fine, but only so long as your style is consistent and doesn't change at random.

This is why pro's put a lot of time and effort into planning their designs, by creating reference sheets to use for themself.

Before even starting with your manga, you should draw all important characters, weapons and such over and over again, from different angles and in different states.

This is incredibly important, because drawing manga pages takes a long time and without a constant reference, you'll be at a risk of slowly altering your designs over time, without noticing it.

Reading takes place at a much faster pace than drawing, so subtle changes in the way you draw a character are often much more evident when you read a manga than when you draw it.