ILR Card
Once you have ILR (before citizenship), you receive Biometric Residence Permit (BRP) or Immigration Status Document (ISD). This is your permission to be in UK.
Traveling Outside UK
You still need passport from your country of origin. Passport + ILR document = proof of right to return. Without ILR doc, UK border may deny reentry even with passport.
Returning to UK
With ILR document + passport: guaranteed reentry (as long as doc not expired). Can travel freely to EU, visa-free countries. Must return before ILR expires.