The poem uses parallelism three times: first to create a list and then to create two antitheses. Together, all this parallel phrasing lends the poem a feeling of monotony that evokes the repetitiveness of the speaker's life.
First, lines 3-4 present a succession of phrases beginning "The room" or "The [...] room":
The room in Paris, the room at Geneva,
The little damp room with the seaweed smell [...]
This structure (which is also an example of the device anaphora) allows the speaker, who is reminiscing about "Rooms," to offer a series of examples in a clear, logical fashion. Line 6 then contains a minor instance of parallelism: the antithetical phrase "for good or for ill." This is an old-fashioned, but still idiomatic, version of the expression "for better or for worse."
In both of these examples, parallelism creates the sense that specifics don't really matter to the speaker. The speaker is simply slotting different locations and scenarios into the same linguistic structure; whether the room was in Paris or Geneva, and whether good or bad things happened there, readers get the sense that it's all the same to the speaker. This person's life has felt claustrophobic and disappointing time and time again.
Parallelism returns at the very end of the poem, in the form of two phrases beginning with "in the" and set off by a dash:
[...] Out there in the sun—in the rain.
Again, parallelism allows for a direct antithesis, or juxtaposition of opposites. In the "quieter, dustier bed" of the grave (line 9), the speaker and their roommate will sleep through both good weather and bad. The world will still have its ups and downs, but they won't matter anymore. Again, parallelism suggests that it's all the same to the speaker.