SF is a dedicated audio-editing program. SONAR has some audio editing features, which may suffice for some projects. SF simply has more.
I use SF frequently for:
(1) Fine-grained audio splicing
(2) Loop creation for use in samplers
(3) Noise reduction when importing audio from old cassettes or LP's
(4) Diagnostics (frequency analysis, location of isolated clips, etc.)
SF also has a neat clipped peak restoration tool for use on occasional stray overs.
(5) I use various SF effects when the SF effect is what I need. The convolution reverb ("Acoustic Mirror") comes with a huge bunch of impulses. The time- and pitch- shifting tools are pretty powerful. The fades in SF are often more to my taste than those in SONAR. Etc.
SF also allows files to be saved in a way that allows undo even after closing out the program -- I like having the whole history available.