This behaviour makes sense to me. In order for a new filesystem to be usable, certain things have to be written so that it can, for example, store the locations of files. This is what gets done during a format.
So why give you a checkbox at all? I'd say that's only really applicable for old, pre-existing filesystems that are already structurally complete. It's a UX bug that you can un-check it the first time.
- If you're asking the installer to repartition, new partitions are going to need to be formatted.
- If you want to preserve data in a partition but also create new partitions alongside it, you'll need to do that manually outside the installer, preferably with something like GParted. You can then either use GParted to create your installation partitions (to which you select in the installer) or leave the space blank and use the installer's partitioner to install into the free space.
- If you're in any doubt, take a backup before doing anything else.
That this is a SSD doesn't really matter:
- SSDs die slowly, sector-by-sector. All but the first and worst SSDs have wear-levelling handled in their firmware at hardware level. This shunts data to a new sector if another dies.
- All but the first and worst can handle tens of thousands of writes per sector. I'm talking about filling and refilling the entire disk over and over and over and...
- A format isn't writing to every sector. It's only writing to create the file tables. It isn't zeroing out all the data.
People who are churning terrabytes of data on a daily basis might need to care more about their SSD health but a format is just creating an inode table. You've probably written more data to your SSD just posting the question than a format would be.
I think part of the confusion comes from the word "format". The old Windows installers' "format" option was a full, "low-level" format that zeroed out all the data on the disk before creating a filesystem. Ubuntu's is the equivalent of a "Quick Format".