If you'll allow me to oversimplify a bit, you can make a pretty accurate guess about a team's collective fortunes before a season begins, provided you have a decent grasp on that team's personnel/quality of management relative to the rest of the league. In Aston Villa's case it's reasonable to say they're firmly in the middle of the pack, with something like 6-8 teams clearly ahead of them and a similar range of teams below. Over the course of the season, they're going to beat some teams they shouldn't (Chelsea!) and lose to some teams they shouldn't (Swansea :/) and the odds are good it's going to balance out in the end.
You can do the same for pretty much every club up and down the league, but there's enough variance in there that you're going to end up with clumps of teams; City, United, Chelsea (and Spurs, it turns out) at the top or Wigan, Blackburn and QPR at the bottom. The equation will clearly change as the year goes along (it certainly did for Villa last year, as it has for Bolton this year) but the larger point is this; the majority of the time, how well you do against teams that are at or near your level of true talent is going to be crucially important in terms of where you are at the end of the season. That's not universally true but it's a very good rule of thumb, and with Villa in the most congested group of teams in the league (that would be "relatively mediocre" in case you were wondering) there's a pretty significant amount of variance at play.
Still, if teams and managers are judged on where they sit in the table at the end of the season then games like this are vitally important. And if Villa are serious about improving on last season's campaign, it's games like this they need to win. Not only because of the bottom line; in case you hadn't noticed Villa have been abjectly terrible at home this season, never managing to take points off of better teams and dropping points to teams they should easily take care of at Villa Park. That absolutely has to change at some point; you can't be anything approaching a good team if you struggle on your own pitch, and to describe Villa's home performances this season as "Struggling" is far too kind.
One of the bigger stories surrounding Everton as of late (especially stateside) has been the return of Landon Donovan for a two-month loan, almost identical to the arrangement Villa has made with his LA Galaxy teammate Robbie Keane. Unlike Keane, there's actually a point to Donovan being here; he makes Everton a better and more dynamic team, not to an extreme degree but to some degree, and that's important. He's also the kind of player Villa's fullbacks are going to struggle with, and if anything about this game makes me nervous that would be it. It gives Everton a bonafide attacking threat, and when your leading goalscorer is a fullback (even one as threatening from set pieces as Leighton Baines) that's a pretty massive addition.
Still, letdown against Swansea aside, Villa have been playing far better football over the past month or two. Three points from this kind of game would be a pretty big boost, and with a winnable game against Wolves to close out the Premier League schedule for January ahead, Villa could go into a pretty tough February in decent shape. A draw and the status quo is maintained, a loss and people start to get even crankier. That would be true no matter the team, but there aren't many teams where any of the three results look equally likely. We think of the big games as the ones coming at crucial times in the season or against the bigger clubs, but in reality this game against a non-sexy Everton in mid-January could ultimately end up being one of the season's more important contests.