As @swbarnes says:
Every time you expose a population of bacteria to an antibiotic, you
select for organisms with resistance-granting genes.
But you're asking about the situation where you're not 'sick' when you're taking them, and how it affects future infections.
To understand why that's a bad thing, you need to understand a couple more things. Firstly, your body contains a massive number of harmless bacteria. Some of those are the same species as harmful bacteria - for example, your gut contains harmless Escherichia coli, even though some strains are highly pathogenic. Secondly, bacteria exchange genetic information.
When you take antibiotics, the harmless bacteria in your gut are put under enormous selection pressure for resistance. This will increase the frequency of any resistance alleles present in the population. If you then catch food poisoning, the 'bad' E. coli will exchange genetic material with the harmless ones, and some of that may include the genes conferring increased resistance.
So if you take antibiotics for no reason (or give them to animals as a growth promoter, for example) you're putting bacterial populations under selective pressure to develop resistance, which can spread to other strains or even other species, and means more bacteria that are antibiotic resistant.