A connecting flight that changes terminal — landing at T3 and departing from T5, for example — is one of the situations where knowing both terminal numbers in advance matters most, since the transfer time between them is a genuine factor in whether a tight connection is realistic. If you are being collected partway through a connection, or dropped to continue a journey by rail after we bring you to one terminal, telling us the full itinerary at booking means the pickup or drop-off point matches exactly where you actually need to be, not just the airport in general.
For anyone unfamiliar with Heathrow's layout, it is worth remembering that T2 and T3 sharing the Central Terminal Area is the exception rather than the rule — T4 and T5 are genuinely separate buildings with their own access roads, and treating all four terminals as interchangeable is the single most common assumption that catches out first-time visitors booking their own transport.