When you say "attend", it could mean 'being present at something' (social gathering, reunion, church services, etc.) example: i attend mass rugularly. My cousin seldom attends our yearly family reunion.

But when you say "attend to", it could mean 'paying attention to something - seriously' example: would you please attend to your responsibility as Sales Supervisor? "attend to" may also mean: 'dealing with something' example: we are obliged to attend this serious matter.

Bless You!