I think Node.js can be used to solve a lot of problems almost all. The question, however, remains which problems does it solve better than others while for which does it do worse than others.
Node.js is best suited where there is a lot of input/output.
Heavy read/writes to databases.
A lot of file handling.
Or as an orchestration middleware to route/aggregate APIs from multiple services.
I think node.js comes of worse while doing heavy computation, calculations or things like image processing.
P.S - The subject and the description of the topic ask different questions respectively. Subject asks for a “Why” where as the descriptions asks for “Where”.
My answer is more focused on the where part.