As per wikipedia: Pi
For most numerical calculations involving π, a handful of digits provide sufficient precision. According to Jörg Arndt and Christoph Haenel, [as you point out,] thirty-nine digits are sufficient to perform most cosmological calculations, because that is the accuracy necessary to calculate the volume of the known universe with a precision of one atom.
Despite this, people have worked strenuously to compute $\pi$ to thousands and millions of digits. This effort may be partly ascribed to the human compulsion to break records, and such achievements with $\pi$ often make headlines around the world.
(This obsession with/compulsion to memorize/calculate more and more of the digits of $\pi$, may also, for at least a few, constitute a manifestation of OCD, and provide grounds for such a diagnosis!)
(To the credit of $\pi$ and its digits) They do have practical benefit:
... such as testing supercomputers, testing numerical analysis algorithms (including high-precision multiplication algorithms); and within pure mathematics itself, providing data for evaluating the randomness of the digits of π.
...[and can be applied to test the accuracy and]
...the "global integrity" of a supercomputer. A large scale
calculation of pi is entirely unforgiving; it soaks into all parts of
the machine and a single bit awry leaves detectable consequences.