What language are we talking about? In languages with more flexible word order, inflection is necessary for meaning.
In Latin:
Puer puellae rosam dat.
Puer rosam dat puellae.
Rosam puellae dat puer.
All of these mean "the boy gives the girl a rose." If the inflection of any of the words were to change, it would alter the subject, direct object, indirect object or tense/person/voice of the verb.
In languages in which word order came to replace inflection, the endings became superfluous and dropped out. This happened in Arabic (in which the case endings have basically been removed), in Hebrew (in which some would argue that they never existed, but we should surely understand them as having been there originally) and in English (where we have only the pronouns as remnants of the old system). If inflections were not obligatory, they would eventually fall away through use.
