Actually, in later years the internal reasoning has been published, so we no longer need to speculate. (This following quotes are taken from the book "Energiya-Buran, the Soviet Space Shuttle" by Bart Hendrickx and Bert Vis).
And the actual reason is even more hillarious.
So, first, we now know the US space shuttle was the outcome of some bureaucratic processes in the US (NASA wants a new program to replace Apollo, the Air Force accidentally ends up in a super good negotiating position, Nixon does not want to be responsible for ending manned flight but also doesn't want to pay for a trip to Mars), so in the end it didn't end up a very sensible design. But the Soviets did _not_ known this, and assumed there must be some secret rational reason for building it:
> As TsNIIMash [Central Scientific Research Institute of Machine Building] director Yuriy Mozzhorin later said: "[The Space Shuttle] was introduced as a national program, aimed at 60 launches per year ... All this was very unusual: the mass they had been putting into orbit with their expendable rockets hadn't even reached 150 tons per year, and now they were planning to launch 1,770 tons per year. Nothing was being returned from space, and now they were planning to bring down 820 tons per year. This was not simple a program to develop some space system ... to lower transportation costs (they promised they would lower those costs tenfold, but the studies done at our institute showed that in actual fact there would be no cost savings at all). It clearly had a focused military goal".
So what might this secret military goal be? Apparently the Soviet speculations was that either it might be part of some nuclear fractional-orbit bombardment system, or it might be part of a plan to launch lots of laser weapons into orbit. In any case, we can't have a space shuttle gap, so:
> "[Central Committe Secretary for Defense Matters, Dmitri] Ustinov had made the following argument: if our scientists and engineers do not see and specific use of this technology now, we should not forget that the Americans are very pragmatic and very smart. Since they have invested a tremendous amount of money in such a project, they can obviously see some useful scenario which is still unseen from Soviet eyes. The Soviet Union should therefore develop such a technology so that it won't be taken by suprise in the future".
Let's build it now, and see what it is good for later! And then the Americans started speculating about what the secret military purpose of the Soviet shuttle might be! :D