What phrase indicates the reason for writing to someone?

The phrase "on behalf of my client" clearly indicates the reason for writing to someone, as it suggests that the writer is acting as a representative or spokesperson for their client. This phrase communicates that the writer is addressing the recipient not for personal reasons, but to convey messages or actions that concern their client. It establishes the context of the communication, implying that the writer's purpose is to represent the client's interests or needs.

The other phrases do not convey the same level of representation. "For my own purposes" is self-referential and suggests the writer's own motivations rather than a client-focused reason. "In support of my client" could imply assistance but does not specifically indicate representation in the way that "on behalf of" does. Similarly, "for the benefit of my client" suggests the intention of helping the client, but lacks the clear, direct portrayal of acting as their representative in the communication.