It's largely true that until about 1918 the name Belarus effectively was a geographic term, and not one that represented a people or nationality. However starting in the late 19th century there was a burgeoning movement to create a Belarusian national identity, though it was fragmented and largely coopted by other groups in the region for their own purposes.

Jerzy Borzecki wrote about the formation of the Belarusian national identity (well technically, "Concepts of Belarus Until 1918") for his MA thesis, which I have a copy of and am using here. His conclusion states that because of the rising surge of Belarusian nationalism, which was exacerbated due to the Russian Revolutions and the general disorganisation of the era, it was impossible for any one country to re-incorporate the Belarusian people as their own, and had to acknowledge that Belarusians were indeed distinct. That a Belarusian state was nominally formed in the aftermath of all this also complicated things, and while it didn't survive long-term, it set a precedent that was hard to ignore.

But as his thesis ends in the fall of 1918, prior to the October Revolution, allow some educated speculation, with the note that I can supply some further reading. The Bolsheviks had a complicated nationality policy, and it changed multiple times over this era. So it's not ideal to say anything with certainty, but it is likely that in order to secure the Belarusians support, they allowed the idea of Belarus to exist in order to bring that region into the Bolshevik's world, and later the USSR. Note that Ukraine was also allowed to remain "independent," even though the Ukrainians were also considered nothing more than a subgroup of Russians (an oversimplification I know).

Part of the nationality policy of the time was to promote nationalism, as though it was derided as a bourgeoisie concept, it was deemed necessary for the advancement to true socialism, especially for those peoples deemed "backwards" or "uncultured," which mainly meant anyone not Russian. Thus Belarusians were promoted as an ethnic group throughout the 1920s while the korenizatsiia policies (promotion of non-Russian groups, a kind of affirmative action) were in place.

So it was a couple factors that led to Belarus being formed, a mix of the growing idea of Belarus as a national identity, the formation of a rudimentary Belarusian state in the aftermath of the 1917 revolutions, and the Bolsheviks desire to promote ethnic minority groups.

As noted, Borzecki's thesis explores Belarus prior to 1918, while I'd also suggest trying to get a hold of The Rise and Fall of Belarusian Nationalism, 1906–1931 by Per Anders Rudling. It should give a fuller view and incorporate the post-1918 era in a more detailed manner. And while I can't say for certain, I'm pretty sure Timothy Snyder's The Reconstruction of Nations: Poland, Ukraine, Lithuania, Belarus, 1569–1999 should have some further information. I just haven't read it in a few years now, so can't confirm that assertion.

I've read both Rudling's and Snyder's papers, as well as papers by N. Becus and P. Tserashkovich on Belarusian nationalism history. It's one of my favorite topics. In general, it's rather complicated even completely understandable and not a unique phenomenon.

As of Soviets, by 1917 Belarusian nationalism was already created even not recognized. There were two major Soviet national concepts - Lenin's and Stalin's. Lenin had an idea of creation a Federation of a Soviet nations (future USSR), while Stalin was against as on Communism stage Nations should be a history and there were no reasons to split Russia. Lenin's conception got a priority and Bolshevicks decided to support national movements and create a national states led by local Bolshevicks.