In the UK, parish registers only begin in the early 1500's, and civil registration did not come into effect until 1837. Not all events were recorded in the parish registers, for various reasons, and many of the registers have not survived. The ones that do survive are held in archives all over the UK, and so, in practical terms, are not really accessible to everyone. The Civil Registrations are comprehensive, but to access the actual primary source, which is to say a birth, marriage, or death certificate, is prohibitively expensive. The situation is similar in other countries.
So where does that leave us?
Since we have limited access to primary sources, we rely on secondary sources and "preponderance of evidence" to make decisions. Secondary sources are typically one step removed from primary sources, such as parish register extracts (transcriptions of actual parish registers — mostly accurate but subject to transcription error) from the IGI and other sources like Phillimore's transcripts; Civil Registration Indexes, census transcripts, and written and oral family histories. Preponderance of evidence simply means that we take 2 or more secondary sources, and if they agree, we accept whatever it is they are telling us as fact.
Where this comes into play most often is in determining birth dates. Often we will know some or all of the children from a given marriage from an informal family history, say, but we may know neither the birth order nor the birth dates of the children. We probably will know the approximate time frame within which the family existed, and then maybe we can find them in the census where their ages are most always given, so we can work out their approximate birth year by subtracting the given age from the census year. That age may or may not be accurate, so now maybe we can find them in a parish register index or civil registration index — the right name in the right place in the right year. So, without ever consulting the primary birth record, we can be quite confident that we've got the correct year of birth, at least.