Full Idea
P→◊P is usually considered to be valid, but its converse, ◊P→P is not, so (by Frege's own criterion) P and possibly-P differ in conceptual content, and there is no reason why logic should not be widened to accommodate this.
Gist of Idea
Modality affects content, because P→◊P is valid, but ◊P→P isn't
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.2)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.5
A Reaction
Frege had denied that modality affected the content of a proposition (1879:p.4). The observation here is the foundation for the need for a modal logic.