The main thing Tumblr offers that I haven't seen anywhere else is curated submissions. Like the sister reply said, on Tumblr, you can allow anyone to submit posts to your blog (including anonymous users and users without Tumblr accounts). You can set restrictions, such as "only text post or photo+caption submissions", and provide tags for submitters to choose from (e.g., on my blog submitters can choose the tag for the TTRPG they're using).
The key differentiator is the curation. Only those submissions which my mod team approves get posted to the blog, unlike Reddit where all submissions get posted first and then up/downvoted accordingly. I think (?) Reddit might have a way to curate, but it's not feasible at the scale of Reddit and a subreddit equivalent of the blog would quickly overwhelm the mod team and have to become uncurated. Considering the number of low-quality submissions we actively curate out, the result would be a significantly degraded experience. Which is not to say a transfer to Reddit is impossible, but it would definitely be An Effort, and one I'm not sure I'm willing to put in for what's been a fun hobby project for the last seven years.
Asks are another Tumblr feature that I haven't seen in other places. An ask is a question submitted to the blog mods, which the mods can reply to either publicly or privately, and which are visually differentiated on users' dashboards. On Reddit, a question to the mods would look exactly like any other post, and the mods' answer could easily be lost in the sea of general comments on the post.
There are also lesser features that Tumblr has that I've never seen on other platforms. Tagging is a big part of Tumblr culture and something I've had a lot of fun with on my blog. Reblogs and replies are very different forms of interaction than retweets/@'s. Fun coincidences like "dash did a thing" (where two unrelated posts show up together on a user's dashboard in a way that's amusing) don't appear to be a thing on other platforms for various reasons. The visual layout and formatting of Tumblr posts on the dashboard tends to be more readable than Reddit or Twitter.
These are all little things, but they're little things that have built up the personality of the blog over the years. I'd rather close the blog with its personality intact than watch it slowly die as yet another subreddit plagued by low-quality posts, insufficient moderation, and sameness.
(This sounds like I'm down on Reddit, which is not the case at all - I like Reddit, just not as a possible host for my blog.)