Well Boullee was an enlightenment man, and Sir Isaac was regarded as a genuinely legendary scientist by the enlightenment thinkers.
It was intended a monument to enlightenment thought itself for one, but too specifically to Newton's contributions to the field of the physics, hence its spherical shape.
>really dumb and expensive for no reason at all
As it goes for nearly every unbuilt architectural marvel.
A tomb may be more useless than the Palace of the Soviets, but the Palace too is needlessly excessive when multiple smaller scale buildings can not only do the job just as well, but better.
(And in fact did do so, since the Palace wasn't built and the Soviets still handled governmental functions fine with other less excessive buildings)