The connectorised cable from a CBT to the CSP is an external grade cable and constructed with regard to the ‘external’ environment it will exist in , as well as the type of stress it will be placed under ( strong enough to be strung between a wall bracket and house wall for example if used overhead , resistant to rodent damage if used underground ) , this cable isn’t designed to be ran ‘internally’ , internal optical cables have a construction that is designed for a different environment (to an external grade cable ) like lower , less toxic smoke should it be set on fire, more malleable , easier to run internally, so it’s necessary to have two different grades of cables , that means there needs to be a point at which these cables are ‘spliced’ together , hence the CSP , and although it’s called a consumer/ customer service point , it can also be regarded as a customer splice point.
In a new build property , the ‘internal’ cable is installed at first fix , so it’s hidden when the internal wall surfaces are completed, this requires the builder being supplied ( by OR who pay the builder for this work ) with internal optical cables that are left coiled at the external wall and the location of the ONT , ready for OR to connect ….having a totally different set of cables for new sites and retro fit sites makes no economic sense , it would cost more to produce and store different types of cables , and require different training etc, much simpler and cheaper to use a common set of materials for both situations, new site and retro fit.
FYI , two stage installs are no more common than single stage , in-fact it’s probably single stage that is most common , but obviously neighbours are likely to have the same type of install….if you live in a quite modern building that has a ducted feed , it’s generally a single stage as would be the majority of your neighbours , because their property would also be ducted….live in an older property that isn’t ducted but underground fed , then it’s two stage as before a contractual date is given , OR need to be sure the new cable can be delivered to the house wall….overhead , generally is single stage , unless the householder has altered the building from when the copper dropwire was installed , making access to the existing wall bracket ‘difficult’ , like building a porch on the building restricting access to the bracket, or large trees now restrict the path of an overhead cable .
Single or two stage install is indicated when the property was surveyed , before the FTTP build was started, it’s never been the case that every install was two stage , then single stage added to the process , it’s always been subject to the location
