mongo aggregation difference in match stage using $expr vs not using $expr

Question

I am a bit confused as to when exactly I do need to use $expr inside a $match stage.

For example, when I want to match against a documents property of lets say "fruit" and I do

$match: {fruit: 'banana'} it returns the correct values with only fruits of bananas.

I often times read about using $match in combination with $expr and $eq which results in the same data output.

When exactly do I have to use $expr then if it works without it as well?