- Routines offer a structure within which to prepare for performance.
This is a case of postmodification by infinitive clause (CoGEL p. 1265 § 17.30).
Unlike -ing and -ed clauses, infinitive clauses as postmodifiers in noun phrases allow correspondences with relative clauses where the relative pronoun can be not only subject, but also object or adverbial and, to a limited extent, complement:
S: The man to help you is Mr Johnson. ['who can help you'] 
O: The man (for you) to see is Mr Johnson. ['Who(m) you should see'] 
Unlike ,  can have an optional subject of the infinitive clause introduced by for. Without such a subject,the infinitive clause in  could be understood, according to context, as ' (The man) that you/he/everyone, etc. should see'.
There is not actually a relative clause (for which a subject would be necessary), but there exists a correspondence with a relative clause.
Corresponding relative clause
- Routines offer a structure within which everyone/they/… can¹ prepare(s) for performance. ("Which" is an adverbial.)
¹ Modal added after remark by user John Lawler