Social Security must consider the combination of your impairments when considering your eligibility for benefits. Even non-severe impairments must be considered in combination with your severe impairments. See 20 C.F.R. 404.1523.

This can be very important. Sometimes a seemingly minor impairment can rule out your past relevant work, and move your claim past step 4 of the sequential evaluation process. 

I have had cases where past relevant work was ruled out due to asthma (can't be around fumes), near blindness in one eye (no depth perception for factory work), and moderate anxiety (cannot interact with the general public), when those impairments were combined with the claimant's other impairments. Each of these claims then won at step 5 of the sequential evaluation process, via the medical-vocational guidelines.

Remember, your RFC must include all your impairments, even non-severe ones.