It can be justified it is indeed a very good idea, if you want to publish it through a classic publishing house. This is because publishers prefer to try how well a book sells before committing to a full series, and finding, halfway through, it would be better (from a business point of view) to discontinue it.
I am currently doing this very technique, because I started with a standalone idea, and there are many things left to explain about the world and characters, but I am not a published author yet. A thing I learned and would be worth pointing is that it's better if you have at least an intuition of the direction and/or ending of the series, the most important pieces, so you can hide in your first book some Chekhov's guns for later use, allowing yourself the freedom to play with them and their relevance.
For example, in Harry Potter series, (SPOILER ALERT) concretely in Harry Potter and the Chamber of Secrets (the 2nd in the series), the plot centers on an object (Tom Riddle's diary) which turns out to be of pivotal importance in the series, but until the 6th book you won't suspect its story hasn't finished already.
Here's an article on ways to write a stand-alone book pilot to a series.