Tips on cleaning data using Python and Excel ?

I was wondering what professional data analysts do when preparing data and cleaning it.

Here is what I currently do:

1- remove duplicates

2-handle Null values

3-melt using pandas to make the table cleaner

4-spot outliers and handle them

What more can I do ?

Also I watched someone remove some fields(columns) because they weren't important enough to include, how do people determine what's important to keep and what's not ?

data datascience python tips