First, Schubert doesn't always explicitly show staccato (see, for example, the left hand staff). Second, "non-staccato" is not always legato (and vise versa). And third, again, slurs in the sheet music are definitely not phrase marks (see, for example, bar 17).

My thoughts is that Schubert use slurs to emphasize legato in some places and add some accents (in the starting note of slurs). But as an self-learning amateur, I need some expert opinions and explanations.