I know r/AskHistorians likes long, in-depth answers, but this is a simple question that - for once - actually has a pretty simple answer and I hope the mods don't mind that this answer is only one paragraph.

In history the simplest explanation is not always the correct one, but in this instance it holds true. The Bolsheviks made tackling the low literacy rate among the population a major priority of the new government because the lack of literacy hampered the ability to organize the people they most needed to reach. Approximately one in five citizens of the Empire/Soviet Union were literate. How are you going to get your message out and get the entire population mobilized and moving in the right direction if only one in five people can understand and appreciate the posters and pamphlets you are producing? The Bolsheviks believed that illiteracy, especially among women, had slowed their rise and was an intentional tool of the Tsarist regime to suppress social mobility and reforms. Were they right? Probably in regards to the Tsarist intentions, hard to say about how much more buy-in they could have achieved with greater literacy. What's undeniable though is that it is easier and safer for the person spreading the message to be able to do so via a discrete pamphlet than having to shout in a public place or hold some sort of gathering to reach a large crowd. Literacy rates increased from approximately 20% in 1919 to 89% in 1939, and the party was able to get its message out to the people who they believe needed to hear it most. Certainly it could be debated whether the Bolsheviks were correct in these assessments, but that they believed in them strongly enough to devote at least some resources to them during an on-going civil war is not debatable, and the results are pretty telling.

Sources

Figes, O. (1998). A People's Tragedy: A History of the Russian Revolution, 1891 - 1924. Penguin Books.

Riasanovsky, N. V., &; Steinberg, M. D. (2005). A history of Russia. Oxford University Press.