Datalog is a formalism like relational algebra but additionally supports recursion.
So it can roughly play the role of SQL. Compiler writers have used it to query their symbol tables and identify patterns for example (seems to be the origin of Soufflé, "static analysis").
I believe the reemergence of datalog is at least in part due to compute and memory being more plentiful so it is more affordable and practical to use a declarative query language. A lot of databases (or the part that is relevant for answering a particular query) comfortably fits in a single machine's memory and an expressive query language is fun to use. There is also an incremental evaluation story.
Another potential reason is that the logical data model makes it easy to represent any kind of data without schema changes and such.
A lot of programs end up either doing SQL or something SQL-like. There are therefore many applications of such datalog like languages.
In many places where people use custom "rule languages" they could use datalog instead. (edited: typo & rule languages)