Does anybody know why some journals have "Topical Collections"?
These are just ways to allow folks to find articles of interest within larger (less focused) journals. 

Good examples are topical collections within a larger Mathematical Biology journal like the Bulletin of Mathematical Biology that correspond to subgroups within the society: https://www.smb.org/subgroups/

I think this will become more and more popular as journals get broader and broader.  Think about a journal like Nature Communications that takes science from any field... it's hard to 'browse' this journal in the way that you might browse a journal IN YOUR SUBFIELD, because the vast majority of the stuff is completely unrelated.  In a topical collection however, you might find things of interest.