If you really wanted infinite reaction mass, couldn't you just tell that drink machine thing to make lots of rocket fuel?
While many SCPs exhibit endless mass production, and under seemingly easier to contain conditions,
http://www.scp-wiki.net/scp-109http://www.scp-wiki.net/scp-812the composition of the resulting mass is less interesting, or useful in the scope of establishing human colony sites.
SCP-871 produces "cake."
Cake is, for lack of better descriptor, already perfect biomass in a very low entropy state (contains lots of chemical energy)-- it is immediately consumable, has a high metabolic load value, and it is comprised of a perfect ratio of carbon, hydrogen, and oxygen. The sponge of the cake contains limited amounts of free nitrogen gas, which is also a useful component, as does the limited protein fraction of the cake. (in normal cake, this comes from the wheat flour's gluten, and from the presence of eggs used to make the cake batter. In the case of this SCP, the cake materializes under unknown circumstances. To my knowledge, no genetic testing has been conducted on the cake to determine if it is indeed made from wheat flour, or chicken eggs.)
Due to the low entropy state, it is highly combustible, liberating significant stored energy per gram.
The resulting biomaterial product of human consumption of the cake is useful industrially in many ways, including production of synthetic graphene, synthetic diamond, chemical denaturation to release useful water, hydrogen, and oxygen, as well as production of a wide assortment of plastics.
As such, the material produced by this SCP is vastly more valuable to space exploration and colonization that that of other SCPs.
Additionally, this SCP is able to perfectly replicate itself, including its replicative powers, which is why it has keter designation, instead of euclid. This means that it can be replicated and installed inside many, possibly an infinite number, of space vehicles, or at human colony sites.
If coupled with other SCPs, fully autonomous, yet still "contained" systems can be landed on planets lacking sufficient mass to retain proper atmosphere, such as Mars, and rapidly provide the missing solid mass required to make such bodies permanently habitable. Swarms of such terraforming units could be dispatched to locations of interest to great effect to human kind.
For these and numerous other reasons, scp-871 is ideally suited to the purpose for which it is being proposed.