Grindclock was originally printed in the Scars of Mirrodin block, in which it was a bit better received. It’s an artifact, and there were some decks that could utilize it fairly effectively in Standard at the time. Grindclock has two tap abilities, one to put a charge counter on it, and another that mills a target player’s library X cards, where X is the number of charge counters on Grindclock. It’s become very popular in Commander decks in recent years where mill is definitely a strategy that is quite well supported. But it doesn't fit at all into the Standard environment.
However, let’s look at its Limited implications. It’s only a 2 drop that can tap to gain a charge counter as soon as its played. As in Limited you are typically playing with a 40-card deck rather than a 60-card deck, Grindclock literally does speed up the game clock, often milling cards your opponent will need to win the game into their graveyard before they can draw them. With a card like Mind Sculpt already in Magic 2015, there is already a minor mill sub-theme going on. So in Limited, if you’re going for a control type build, Grindclock is not the worst rare draft in the world, and it’s likely never going to be first-picked.
One cool combination that existed in the Magic 2015 set was with the two-drop Enchantment, Ensoul Artifact, which turns an artifact into a 5/5 creature. A Grindclock that is also a 5/5 creature is pretty fun. But you're not going to build around that interaction.
So is Grindclock sort of a weak rare? It is for many people. But do remember, Core Sets are going to have a few cards for different sorts of players. Grindclock is one of those cards that some newer players or more casual players will want to run and it does have utility in Commander, so it’s not a bad reprint from that perspective. I’m not going to want to open any Grindclocks, personally, but when I draft the set I’m perfectly happy to draft one of the value uncommons in the set over that anyway.
In Standard, mill was not really a thing in the format anymore. Phenax, the God of Mill (technically he’s Phenax, God of Deception) was a potential core piece of a mill deck, but that plan never emerged as viable competitively.
Phenax allows for a lot of card milling to go on., but mill simply never had a deck that actually emerged into a real contender in the Standard meta-game. Gaining a card like Mind Sculpt in Magic 2015, which is a sorcery for 1U that mills a target opponent 7 cards, didn't even help. Consuming Aberration, one of the deck’s main win conditions rotated out of Standard that following October. Ashiok, Nightmare Weaver, an often overlooked planeswalker, which looked to be a strong piece of a mill deck, remained a part of the post-Return To Ravnica block Standard. But it never made a mill deck work, except in Modern where more powerful mill cards exist.
That being said, Grindclock was perhaps the worst rare you could pull from a Magic 2015 booster box… But it kept up a mill sub theme that seems to emerge in a lot of Core Sets. So it made sense, even if it was sort of a seemingly random inclusion in a set meant to make artifacts into 5/5 powerhouses...