steamers were of course important for the penetration of the interiors of Africa, esp on the big rivers such as the Nile, Niger, Congo and Zambezi. They were also very important for helping to improve and maintain communications, not only between colony and empire but also within colonies, especially the large sprawling ones.
At the same time, as steamers are a high demand good for nations in certain points, acquiring them without having the ability to build your own denotes having a degree of prestige and status in the world, thus having them as a requirement for colonization does help reduce the chances that an ahistorical AI colonizing nation gets a colony.
Of course the new system with life ratings introduced by Revolutions helps reduce that chance much further, and also gives a better way to make the colonial race happen at a more appropriate time in game than say at the start. But keeping steamers as a requirement, given both the historical role that steamships played in terms of Europeans gaining mastery over their new colonial possessions in places like Africa and the role in gameplay to help reduce the chance some truly ahistorical nation gets a colony (think of all the complaints EU3 players have had since that games release with odd colonization), and I think Victoria has a pretty good system overall in comparison.
It's not perfect though, and in particular the whole treatment of colonized people in the process of colonization that the Victoria system implies is, to put it bluntly, horrible. Passive spectators welcoming new colonial overlords was not how colonization worked from the point of view of the colonized. Which is why in the VIP mod we've pretty much junked colonization in favor of actual native states that players have to interact with to secure colonial domination if so desired.
