One reason is that, I believe, currently if even a single 32-bit app is loaded on a newer iOS device, all the standard 32-bit libraries have to be loaded into memory as well which causes increased memory pressure. This is a problem given that current iOS devices only have 1 GB of RAM. Getting older apps migrated to 64-bit is therefore a big deal for this reason alone.

There are many ways and reasons why changing your build target may affect your app, so much so that's it's far beyond the scope of a hackernews thread. Supporting 64-bit, although introduced a while ago already with with iPhone 5S, can present some tricky bugs, also too much to explain here.

It's also meta-interesting as it shows the pace at which Apple wants app developers to adopt the new APIs, and will ultimately result in a more modern/ios 8 friendly app landscape.

EDIT: somebody else noted that you can likely still target earlier versions, which on the whole makes this much less interesting.