An indicator is suitable if its color change interval includes the pH at the equivalence point.

• For a Weak Acid vs Strong Base titration (e.g., Acetic acid vs $NaOH$), the salt formed (Sodium acetate) undergoes basic hydrolysis. The pH at the equivalence point is $>7$ (typically $8$ to $9$).
• Phenolphthalein changes color in the range $pH8.3-10.0$ (Colorless to Pink).
• Thus, it fits perfectly for this titration. It is NOT suitable for Strong Acid vs Weak Base because the end point pH would be $<7$.