U.S.S. Mariner has a fantastic post up about why roster management matters.
Each of those decisions isn't that important, and it's rare that teams have a choice between great backup catcher and horrible one, or awesome long reliever or no long reliever, but each of the decisions they make helps or hurts the team, and totaled, can determine a team's success or failure.
Take, for one example, the A's. The A's spend an enormous amount of energy on this kind of thing. What happens if we lose our second baseman? Our outfield defense sucks, what can we do about it, and who's available who might help us? Is it worth it to find a backup corner infielder? Who's interesting on the minor league free agent list, and what'll it take to stash them in Sacramento? Every waiver wire transaction gets looked at by someone, and if they think there's a player available that's better able to help the team than the guy they already have, they'll start that conversation.
Sometimes they suck at it, and sometimes they decide not to take chances, but this is part of why the A's only take so much damage from injuries: they're rarely stuck running out guys who are significantly below replacement level, because they make roster management a priority. Many teams will look at a decision like the backup catcher and go "enh, the kid plays good defense, he's cheap, let's go worry about our rotation". The A's may end up accepting that that kid is the best option for now, but not because they don't care, or because they don't think it's worth their energy.
Go read the whole thing. Clearly, better management of the Pirates' roster will not solve the team's problems, but consider that, by the end of the year, this team had recalled Mike Edwards three times even though Edwards ended up posting a .689 OPS at Class AAA and a .423 OPS with the Bucs. This team paid Jose Hernandez hundreds of thousands of dollars above the league minimum. This team was wasting time with Britt Reames and Juan Perez, only to see one of its minor league relievers, Scott Strickland, signed by a well-run franchise to a major league contract right after the season ended. This team called up non-hitting outfielder Rajai Davis in mid-August even though the team couldn't score to save its life at the time. This team gave roster spots and playing time to minor league nobodies Shane Youman and Carlos Maldonado, and it appears to be going into spring training with those players still on the roster. This team's first move this offseason was to place another minor league nobody, pitcher Josh Shortslef, on the 40 man roster.
None of these things are very important, but taken together, they add up. Do you think the A's would start next season with Carlos Maldonado as their backup catcher? No way. Would they be calling Shane Youman the first, second or third time one of their starters went down? No. The Pirates need to get their hands back on the wheel, and stop filling its last few roster spots with guys who have no upside or talent.