I just finished an "Eastern Modernisation" course focusing on Qing China and Tokugawa/Meiji Japan - in both cases, Manchuria was fought over (see: Russian expansion post-unequal treaties and Boxer Protocol, and Manchurian invasion by Japan in 1931) by various countries. Keeping in mind that the Qing dynasty was declining rapidly throughout the late 19th and early 20th century, why was Manchuria so important? Was it a strategic area? Was it particularly full of resources?
Russian expansion into Manchuria and the Amur River basin during the mid-nineteenth century enfolded a number of ideas and imperatives of Russian colonial expansion that acted to reinforce St. Petersburg's interest in the region. Firstly, there was the idea that both the Amur River basin and the surrounding areas to the south were bountiful with natural resources and would serve a Russia's own version of the Mississippi River- a vast waterway in which would allow Russian trade to flourish. Various imperial geographies highlighted this regions' resources and made explicit connections to the ongoing settlement of the US Western frontier. This was a particularly attractive vision of a modernizing empire, especially after Russia's defeat in the Crimean War and within the context of Alexander II's Great Reforms, a series of mediated reforms. The long historical process of the Siberian fur trade settlement seemed to give a precedent for the successful acquisition and Russification of this territory and added credence to writers arguing Russia had a Manifest Destiny in Northeast China. Although the Amur River basin never became an analogous to the Mississippi as the differences in geography are quite great, this vision was very attractive within governmental circles up through the Russo-Japanese War.
Russian interest in this region was not just the byproduct of imperialist projections. The Russian Army operated on the frontiers in the nineteenth century almost as a quasi-independent force for expanding Russia's sphere of influence and control when the opportunity presented itself. A number of the commanders in the Russian Far East such as K. N Gribskii had begun their careers as junior officers in Central Asia in the 1860s and 70s, where the Russian Army had expanded the empire without the explicit permission from St. Petersburg. After the Boxer Rebellion, many of these officers became insistent that Russian interests had to be protected proactively. This meant occupation of Manchuria and attempts to secure the interest of Russian rail concerns. These officers were also alarmed by Japanese interests in Korea and Manchuria, and thus acted at times without formal authorization.
The relative autonomy of the military on the frontiers reflected the highly divided nature of the Russian government over the direction of Russian imperial affairs. This division was especially pronounced after both the Sino-Japanese War and the Boxer Rebellion. Unrest in Manchuria had led to Russian military occupation. This duration of this occupation posed a quandary for the tsarist government. Members of the Foreign Ministry and the Finance Ministry, like Sergei Witte wanted to use financial power to gain influence within China and return areas in Manchuria to Beijing as part of a larger plan of creating an Russo-Chinese alliance under the leadership of the tsar. For Witte, the expansion of Russian railways into Manchuria were a key component of this restoration of the Qing and would stimulate the economic development of both states. Aleksey Kuropatkin, the War Minister, steered a middle course of keeping Russian troops in key strategic areas, but making large parts of Manchuria into an independent vassal state. Admiral Evgenii Alekseev, commander of the Pacific Fleet and chief officer of Russian troops in the Liaodong Peninsula argued that Russia's imperial destiny was hitched to the Far East and advocated a more aggressive policy. Aleeksev increasingly had Nicholas II's ear with regards to imperial policy, but no single faction had gained significant traction. As a consequence, Russian grand strategy within Manchuria remained in a state of limbo prior to the Russo-Japanese War. Russian troops remained within Manchuria as the Russians had already committed significant resources into this area in the form of railways, even though these seldom reaped much benefit to Russia. Imperial policy fluctuated between extremists like Aleeksev who was a firm believer in the Yellow Peril and Russia's manifest destiny in the Far East, and moderates like Witte that saw that extreme Russian imperial claims diplomatically isolated the empire and reaped few rewards. The consequences of divided colonial policy would be a contributing factor to Russia's defeat in 1905 as the extremists like Aleeksev animated the Japanese to take action to prevent a Russian takeover of the region.
Sources
Bassin, Mark. Imperial Visions Nationalist Imagination and Geographical Expansion in the Russian Far East, 1840-1865. Cambridge: Cambridge University Press, 1999.
Schimmelpenninck van der Oye, David. Toward the Rising Sun: Russian Ideologies of Empire and the Path to War with Japan. DeKalb, Ill: Northern Illinois University Press, 2001.
Manchuria is quite rich in natural resources, especially minerals and timber. For resource-poor Japan(which was struggling even after acquiring Korea and Taiwan as colonies), that's a major plus.
Source 2 this source seems to have most of the country's natural resources down(covers Manchuria/Northeast China specifically as well)
However, I must admit I don't know if this was the motivation for Russia to move into Manchuria. I mean, the country seems to have just as many resources in other parts of Russia.
There is a possibility that Russia wanted to move into Manchuria to extend its influence in Northeast Asia(hence its attempts to grab Korea resulting in the Russo-Japanese war among other reasons).
I don't know how credible this is, but the theory that Russia expands to protect its vulnerable inland areas and gain access to warm water ports(from the CaspianReport videos) might be legitimate as well.