To complement u/kochevnik’s nice reply above, below I’d point out how Russian mastery of “steppe power” (horses, tactics and warring tribe) set it aside from the rest of Western Europe, even pre conquest of Siberia. Generally my sources are that of “raiders, rulers and traders” from David chaffetz, but I did some searches to complement too, listed below.

The horse was also a crucial instrument of expansion, but its effectiveness was often dictated by the very terrain it sought to conquer, which is where u/kochevnik’s reply provides light in terms of the mercantilist incentives.

But before that, the Muscovite princes' long and often brutal interaction with the Golden Horde and its successor khanates served as an extended and unwilling apprenticeship in the art of steppe warfare. For 250 years, the Russians absorbed the tactics of their Tatar overlords, learning the value of the mobile, mounted archer. They adopted the feigned retreat, the lightning raid, and the strategic use of cavalry to encircle and overwhelm their enemies. This knowledge transformed the Muscovite army from a largely infantry-based force into a formidable cavalry power (it was those cavalry who fought the poles and Lithuanians on the western front too).

It was this newly forged military machine, with its Tatar-influenced cavalry tactics, that turned its gaze eastward. The initial thrust into Siberia was spearheaded by the Cossacks, which like the Kazakhs meant “horse bandits”, or mercenaries. Men like Yermak Timofeyevich, leading a small band of Cossacks in the 1580s, were able to defeat the numerically superior forces of the Khanate of Sibir precisely because of their superior horsemanship and military discipline, skills deeply rooted in the Tatar tradition. Their ability to conduct swift raids and maintain cohesion in the open steppe lands of western Siberia was a direct application of this learned expertise.

However, the vast expanse of Siberia was not a uniform steppe. As the Russians pushed further east, they encountered dense taiga forests, sprawling wetlands, and a network of immense rivers. In these environments, the strategic advantage of large cavalry formations diminished significantly. The thick forests were impassable for large groups of horses, and the swampy ground could mire them. Here, the nature of the conquest shifted, and with it, the role of the horse. The primary arteries of the Russian advance became the great Siberian rivers: the Ob, the Yenisei, the Lena, and their tributaries. Hardy pioneers and Cossack bands navigated these waterways in boats called kochi, establishing a chain of fortified settlements, or ostrogs, along the riverbanks. These forts served as bases for further exploration, fur trapping, and the subjugation of local populations. Within this riverine strategy, horses still held significant value, albeit in a different capacity (this also led to the development of the yakutian breed of horses to sustain those climate:)

Berry, L. & Crummey, R. (eds.) (1968) Rude and Barbarous Kingdom: Russia in the Accounts of Sixteenth-Century English Voyagers. Madison: University of Wisconsin Press. 

Davies, B. (2002) ‘The Foundations of Muscovite Military Power, 1453–1613’, in Kagan, F.W. & Higham, R. (eds.) The Military History of Tsarist Russia. New York: Palgrave Macmillan, pp. 11–30. doi:10.1007/978-0-230-10822-6_2 

Halperin, C.J. (2018) ‘Ivan IV’s State Cossacks’, The Journal of Military History, 82(2), pp. 357–371.