The Soviet Luna sample return program - relatively inexpensive as it was -
was mainly designed to beat the USA in terms of bringing home lunar surface
samples before Apollo did, since their own manned lunar ambitions were literally
blowing up in their faces. They later claimed they had no such plans, which
many in the West (especially the media) bought until the Soviet more or less
casually revealed to the world their rather detailed plans in 1989.

Most of the robot landers failed - Luna 15 being one notable case during the
Apollo 11 mission in 1969. If I am recalling correctly, most of Luna 16's sample
spilled out of its scoop as it tried to put the precious cargo in the spherical
return capsule. Luna 20 and 24 were the two other successes of the series,
but as has already been stated, they returned grams compared to Apollo's 800
pounds (but I see in the latest Air & Space magazine that those samples
may have been contaminated due to seal leaks from the nasty lunar dust!).

I actually agree that the right kind of robot lander with current and planned
future analysis technology could do a great in situ job of studying another
world's surface far cheaper than a human mission, but the Soviet Luna
landers - as good as it was to get even a little bit of places no other
missions had explored before or since - were not it.