An SLA turns promises into numbers. For IT support it normally defines priority levels, for example a whole office offline versus one slow laptop, and gives each a target time to respond and sometimes to resolve. It states the hours of cover, how to report an issue, how performance is measured and reported, and any service credits owed when targets are missed.
Without an SLA, "fast support" means whatever the provider says it means. A written agreement lets a small business compare providers on the same terms, plan around realistic repair times, and hold the provider to account with evidence. It also forces a useful conversation about which systems matter most and how long the business can cope without each one.
Check the difference between response and resolution: a response target only promises that someone starts work within the stated time, not that the problem is fixed. Look at whether targets apply at all hours or only in business hours, what is excluded, and whether the credits are meaningful. For our managed IT and on-demand support, support hours and response times are set out in the service agreement.