The concept of time signature is considered as a way to divide a song into smaller equally spaced parts. It facilitates the execution of the music by defining time, pulse and rhythm, practically "organizing" the notes. The vast majority of the blues that we know, has a structure of 12 bars that are repeated. There are also 16 or 8-measure blues, meaning we need 8 measures to develop an idea, 8 measures repeating several times during the song. You may have noticed that in blues and jazz, musicians play solos during the music. This is usually the size of improvisation for each musician, these 12 measures. Or if the blues is 8 measures, the improvisation will have that duration.