The Beetle allows you to put a +1/+1 counter on it each time a player sacrifices a creature. This allows for the Beetle to grow quite large, especially in Commander, where sacrifice themes are rampant. But this is hardly a popular card in Modern, despite there being decks that could take advantage of its ability. It doesn't have trample, though, and it doesn't fly, so it basically just becomes a big dumb beater. Still, it can potentially kill out of nowhere.
For Modern Masters 2017, it was decided to reprint this creature at common. This makes sense, as in Rise of the Eldrazi, the sacrifice theme was extremely important to the set. Having a creature like this at common or even uncommon would have unbalanced what is considered to be one of the best draft formats in Magic the Gathering history. Now being more readily available, it's likely we'll see Mortician Beetle pop up in more Commander decks.
The other impact of this rarity change means that Mortician Beetle is now legal in the all-common Pauper format. While there isn't currently a top deck that could use the Beetle in what's actually an extremely competitive format, Mortician Beetle could see play in new brews based around Tortured Existence, a good card that's been in the format for years.
This is a good reprint, although it's a bit disappointing for people that bought Rise of the Eldrazi foil Mortician Beetles for their Commander decks. Still, this is a card I didn't expect to see at common, and it's hard not to like it at that rarity.