Elizabeth 1 was adamant that her Church records would be the best kept in the World, and she passed an act to have transcripts made of every register in every Parish, 4 times a year, this wasters had an overhaul under an order from Elizabeth the first in 1598, shes done by the Bishop in every Diocese undertaking to visit himself, or send an underling, to organise an exact duplicate copy of the previous 3 months register entries, each type of entry being kept in it's own register, i.e., a baptism, marriage and burial book . These transcripts were then taken back to the Diocese and kept in considerably better surroundings, and this is why the Bishop's Transcripts (BTs) on the whole survived down the centuries, in many cases they are the only surviving registers in some Parishes. Most BTs are now deposited and filmed in CROs, these are often the ones that people get to look at, and they can usually be indentified as BTs because of the repetitive nature of the entries.
The IGI (International Genealogical Index) was a project begun by the LDS Church (Mormons) 50 odd years ago. It's purpose was and is, to identify the ancestors of it's Church Congregation and then postumously baptise those ancestors into the Mormon Congregation. The resulting records are massive, however, to be able to film the vast archive of parish Registers, the Mormons had to obtain permission from the Bishop of every Diocese, and certain Diocese reused that permission, hence there are huge gaps in Certain Counties, e.g.Cheshire. As the original microfiche records were published in County order this was more noticable, now in the online form at familysearch.org., it is not as obvious.
It is also worth noting that the IGI mainly only contains the Parish Records of the Church of England in England and Wales. It does not use many catholic or non-conformist records. These are registers that would need to be sought at CROs in addition to the entries on the IGI.