What is the practical usage difference between "grep -e" and "grep"?

I have read the manual, but still I am not sure how it works. It says

-e pattern, --regexp=pattern

Specify a pattern used during the search of the input: an input line is selected if it matches any of the specified patterns. This option is most useful when multiple -e options are used to specify multiple patterns, or when a pattern begins with a dash (‘-’).

But I don't understand how this is different from any regular grep without -e.

Here is the code I used to figure that out:

apple@apples-MacBook-Pro ~ % cat testgrep
warning
critical
error
passage
warning|critical
apple@apples-MacBook-Pro ~ % grep -e "warning|critical" testgrep
warning|critical
apple@apples-MacBook-Pro ~ % grep "warning|critical" testgrep
warning|critical
apple@apples-MacBook-Pro ~ % grep -e 'warning|critical' testgrep
warning|critical
apple@apples-MacBook-Pro ~ % grep 'warning|critical' testgrep
warning|critical
apple@apples-MacBook-Pro ~ % grep -e "warning\|critical" testgrep
warning
critical
warning|critical
apple@apples-MacBook-Pro ~ % grep -e 'warning\|critical' testgrep 
warning
critical
warning|critical
apple@apples-MacBook-Pro ~ % 

As you can see, they seem to behave in exactly the same manner. Is my understanding correct that they are pretty much the same thing except you can use -e to search multiple patterns at the same time, just like you can with the | operator?

Solution 1:

When I run

 grep -e "search1" -e "search2"

Grep searches for both term 1 and term 2 (ie it returns lines matching either search 1 or search 2 or both), where as running the same command without -e errors - ie the -e allows for multiple different patterns to be specified seperately.