The included tools share the same code as it is used in the ACPI
implementation of the kernel. The code of the acpica project is exactly
the same as the ACPI parser and interpreter code of the kernel and the
code gets synced regularly from the acpica project into the kernel.
E.g. if you identify bugs in the kernel's ACPI implementation it might
be easier to debug them in userspace if possible. If the bug is part of
the acpica code, it has to be submitted to the acpica project to get
merged into the mainline kernel sources.

iasl compiles ASL (ACPI Source Language) into AML (ACPI Machine
Language). This AML is suitable for inclusion as a DSDT in system
firmware. It also can disassemble AML, for debugging purposes.