CG13220 - Value shifting: TCGA92 S29: introduction
Without special rules, it would be possible to manipulate assets such as shares, or interests in land, so that value passes from one holding or interest into another without there being any disposal for capital gains purposes. The value shifting provisions in TCGA92/S29 were introduced to counter such avoidance.
You are most likely to see cases where these rules apply in transactions between connected persons, for example transfers of value from parents to children, or between associated companies.