So, ok, I shouldn't, but I'll bite. Who is taking Weapon Master, and why would they prioritize it?
1. Bladesinger: Human or Custom Lineage Bladesinger at 1st level any another race Bladesinger at 4th level that did not get weapon proficiencies with their race.
Either case, bump Dexterity and get proficiency in hand crossbow (the only decent missile weapon you can use in bladesong) and 3 of the following: Scimitar, Short Sword, Rapier, Whip .... take 4th weapon at 2nd level when you get your subclass. This gives you all the weapons you will want to use with Bladesinger extra-attack.
This feat is pretty mandatory on a melee-optimized Bladesinger if you are not playing a race that gets you weapon proficiencies, or even if you are and that race is anything other than a Drow and your DM won't let you switch the proficiencies.
2. Rogue: On a human or custom lineage Rogue at 1st level to bump dex and get a heavy crossbow (highest damage sneak attack weapon), whip (only reach melee sneak attack weapon) and Scimitar ...... and probably Longbow (no real reason but you have 4 weapons).
3. Monk (Situational): Monk when you found an awesome weapon they are not proficient in that they can use dedicated weapon on. Say for example a Vorpal Scimitar, Blackrazor, Wave, Ironfang or Sword of Zariel. Even something like a Flametongue or Dragon Wrath weapon not on the Monk list can be a big boost to damage for a Monk and worth the feat and finally at high level the Monk can use a whip with reach using a martial arts die instead of 1d4.
I have done both #1 and #2 in play on Variant Human characters