Since the dissolution of the USSR in 1991, lot of its republics chose to be independent states and most succeeded without any significant pressure from the then newly established Russia. But one particular republic wasn't successful at all, which is Chechnya.
Chechnya's retention history has multiple war campaign marks. Litvinenko's controversy also suggests that Russian apartment buildings bombing in 90s, allegedly performed by the government of Russia, were done for the purpose of a justifying of the war against Chechnya. Lot of financial and personnel resources were thrown on the keeping Chechnya a Russia's republic.
Even now, when there are no any major war escalation in the region, Russia allegedly spends a lot of money to keep it in control.
There are several claims that suggests that Chechnya is critical to Russia.
Rich oil deposits.
Although Chechnya may be a major hub in the oil infrastructure of Russia, one can assume that more resources were spent to prevent its secede. And still, Russia is notoriously rich of natural resources, even without Chechnya, to the point of its economy dubbed resource-based, so the claim that Chechnya's oil is so important for Russia is rather doubtful.
Some claims that Chechnya has strategically important location. However, this is also questionable, since Chechnya doesn't have a sea exit and has no exclusive border with adjacent countries. In fact, at the south it has a small border with Georgia, but adjacent Russia's republics' Dagestan, North Osetia, Kabardino-Balkaria and Karachay-Cherkessia aggregate borderline with Georgia is much longer.
Secede case for other republics.
Boris Yeltsin's claim, who was a Russia's president during the escalation of this conflict in 1992, was based on a belief that other federal subjects, such as Tatarstan may follow the example of Chechnya and try to secede. In fact, all of these subjects have already signed their stay treaties, even Tatarstan which made it last, and it is very dubious, that if Chechnya was granted independence, they may have change their mind.
So what may be a real motive to keep Chechnya within the Russian Federation?