While the reason behind the Jedi tomb wasn't revealed in Obi-Wan Kenobi, Star Wars: The Rise of Skywalker and The Mandalorian hint at the Empire's sinister motives.
Obi-Wan Kenobi may never explain why the Empire keeps a Jedi tomb in the Fortress Inquisitorius, but it serves as a brutal reminder of the Empire's injustice. The Empire may be holding on to the younglings killed in the Clone Wars since the Inquisitors' main goal is to wipe out the Jedi. In a chilling scene in Obi-Wan Kenobi episode 4, Obi-Wan "Ben" Kenobi stumbles upon a sub-level of Fortress Inquisitorius, which holds rows of Jedi corpses in tombs. The experimentation was never stopped even after Return of the Jedi and Obi-Wan Kenobi were set five years after that. Obi-Wan discovers the Empire's secret collection of dead Jedis while Reva and Obi-Wan are in the Fortress Inquisitorius. After the events of Star Wars: Return of the Jedi, Palpatine's spirit was transferred to a clone body. It's possible that the Jedi corpses that Obi-Wan discovers were used as Imperial test subjects were also being evaluated for midichlorian count. The bodies range from familiar to obscure, including a youngling slain during Order 66.
The Empire's interest in Baby Yoda/Grogu and his blood was emphasized by the Mandalorian. In Star Wars: The Rise of Skywalker, Palpatine had a laboratory on the planet Exegol which housed Snoke clones. It's not clear how many Jedi are in that room, but it's a somber memory of Obi-Wan's past. It doesn't clarify the expressions of shock on their faces, but it may explain why the bodies look so well taken care of. The Jedi tomb could be connected to Palpatine's search for eternal life.
He was going to rescue Leia for the second time from the Third Sister. The Empire has a tomb of dead Jedi bodies. The Jedis were killed in order to keep the galaxy from becoming a full-blown rebellion. The Empire wanted Grogu's blood to create more Force-sensitive users. It's not clear what purpose these bodies serve for the Empire. The chance that they could be useful in future experiments is why their bodies are being preserved. The answer may be found in other Star Wars series and films.
There are new episodes of Obi-Wan on Disney+. After Palpatine's death, he was still intent on immortality. Emperor Palpatine's quest for immortality is the most likely reason. There are failed experiments in an Imperial lab.