Yes, online information about partition alignment is confusing. Neither Wikipedia nor the top Google results as of today do a decent job at explaining the benefits. ChatGPT however, says this (which makes sense but I haven't fact checked it):
Partition alignment is important for SSDs because of the way they store data. SSDs use a type of memory called NAND flash memory to store data. NAND flash memory is organized into blocks, with each block containing multiple pages. When data is written to an SSD, it is written to one or more pages within a block.
If a partition is not aligned with the underlying block boundaries, then the SSD may have to read and write data across multiple blocks. This can lead to decreased performance and increased wear on the SSD. In some cases, it can even cause data corruption.
To avoid these issues, it is important to align partitions with the underlying block boundaries of the SSD. This can be done by creating partitions that start at a multiple of the block size, which is typically 4KB or 8KB for modern SSDs. Many operating systems, including Windows and Linux, will automatically align partitions correctly by default
Further, it also explains why sometimes there's a bit of unallocated space after the last partition:
there can be benefits to leaving unallocated space on an SSD, especially at the end of the drive. This is known as overprovisioning, which means leaving some of the NAND flash memory unused by the file system.
Overprovisioning provides a number of benefits, such as improving performance and increasing the lifespan of the SSD. By leaving unallocated space on the drive, the SSD has more space to perform wear-leveling, which is the process of distributing writes evenly across all NAND flash memory cells to prevent any one cell from wearing out faster than the others. This can help to prevent performance degradation and extend the lifespan of the drive.
In addition, overprovisioning can also help to improve write amplification, which is the ratio of data written to the drive versus the amount of data that is actually stored on the NAND flash memory. By leaving some space unused, the SSD has more space to perform garbage collection and consolidate data, which can help to reduce write amplification and improve overall performance.