why databases use b+ trees to hold data

so we all know most databases store data in b+ trees, but why ? not just sql databases either - even non-relational databases like mongodb leverage b+ trees to store their data. mongodb's storage engine wiredtiger serializes collection data as b+ trees on disk. but let me tell you why there was even a need to introduce a data structure like b+ trees in the first place, and how this actually works…