Halliday & Matthiessen (2004: 576-7, 578):
At the same time there are other instances of lexical cohesion which do not depend on any general semantic relationship of the types just discussed, but rather on a particular association between the items in question — a tendency to co-occur. This ‘co-occurrence tendency’ is known as collocation. … In general, the semantic basis of many instances of collocation is the relation of enhancement …
… collocation is one of the factors on which we build our expectations of what is to come next. … collocations are often fairly specifically associated with one or other particular register, or functional variety of the language. This is true, of course, of individual lexical items, many of which we regard as ‘technical’ because they appear exclusively, or almost exclusively, in one kind of text. But it is also noteworthy that perfectly ordinary lexical items often appear in different collocations according to the text variety.