Differences between HMAC and HKDF in a specific case
HKDF stage 1 is useful for when the SKM value is derived using some deterministic method, such as a key exchange, or from a source that may not be fully trusted. SKM may be substantially larger than the keys you want, but may have the entropy not evenly distributed. Stage 1 compacts and distributes the entropy into a key that is correctly sized for the hash function in stage 2.

If you have a source key from a true random number generator, or that has already gone through a strong KDF, you can substitute PRK with SKM, and skip the first HMAC operation.