I don't know about you but I would be hard-pressed to think of any meal that didn't make use of some form of seasoning to enhance the often bland flavor of a base meal. Think about everything you eat and I'm willing to bet the vast majority of it has been seasoned, from soups and stews to your grilled steak and even eggs.
Obviously, nearly any meal you make can be eaten without seasonings and I'm willing to bet that if you had to you would--well, maybe not--but if you don't have to eat a bland meal, why?
The question is, why does it matter?
I for one will eat darn near anything, from obviously bland to overly-salted, I don't care. My family, on the other hand, won't. They like taste and who can blame them. We're all accustomed to flavors in our foods... yes, we're spoiled! That said, they don't like things spicy but definitely not bland. I can understand and I'm sure you can too.
But, the reason why I think spices and seasonings are very necessary is the simple fact that our diets will likely contract quite a bit in any true long term grid-down situation. Granted, we'll want to add variety to meals but eating the same meal over and over again (often including much more of bulk foods like rice and beans) gets old, even for me. We NEED a way to mix things up and seasonings do precisely that. We know this instinctively and must plan for it logically.
In fact, there's something called "appetite fatigue" where a person simply won't eat food because they don't like the taste, eat it over and over again, or it could be a result of medications too. It can be so much of a problem that a person may actually starve themselves to death as a result. I think that's crazy talk but what do I know? Regardless, if I can help prevent this potential problem I'm going to.
What seasonings am I talking about?
Typically, when we think of seasonings we think of the stuff that sits in the cupboard next to the stove (or wherever you keep yours) and include both spices and herbs, such as garlic powder, cinnamon, chili powder, onion powder, nutmeg, cloves, cumin, ginger, and, of course, salt and pepper. There are also plenty of blended seasonings too, such as nature's seasoning and Cajun seasoning to name a few. Likewise, there are seasonings meant for specific purposes or foods, including poultry seasoning, Mexican seasoning, and creole seasoning as examples, and which are also blended. Last, are an assortment of seasoning packets for specific purposes too (such as taco or fajita seasoning). Basically, anything that is in dried form is a likely candidate.
Generally, however, the vast majority of seasonings mentioned above (as well as many other herbs and spices) are meant for dinner meals. What about breakfast foods? Well, these are usually much easier to prepare for. Honey would be one great prep to stock that can be used as a sweetener for so many meals and foods.
What I'm not talking about
Things like marinades and sauces for meats would be prime examples as I doubt they keep very well for the long term. That said, I know there are plenty of good rub recipes out there that can be fashioned from an assortment of herbs and spices. In this case, print out a few of your favorites and ensure you have plenty of those seasonings on hand. Other "seasonings" such as ketchup, mustard, and other condiments are best made from scratch when needed.
How to keep seasonings long term
In my opinion, most herbs and spices last for a long time just letting them sit out, but I 'm sure that depends on your climate too (with humidity being the biggest problem). As for how to store them, do it like any other long term prep: mylar and oxygen absorbers. I've done this with success but have recently switched to using my trusty foodsaver instead and without an oxygen absorber. If I expected to keep them for 30+ years then it would be mylar and o2 absorbers. Anyway, I use the foodsaver because I can see through the clear bags but that's just a personal preference. In fact, you could store a whole array of spices in a five gallon mylar bag if you wanted.
There's a reason why Roman soldiers were paid a wage in salt... not only was it necessary for life but made things tasty too.