A couple of weeks back I posted a 6 question quiz about medieval cooking. I had tried to phrase the questions so that there would be many possible answers that could be considered to be correct depending on viewpoint. Here are my thoughts on the second question.
2. Why did medieval Europeans use a lot of spices in their cooking?
The answer to this question really depends on how the phrase "a lot of spices" is interpreted. It could be understood to mean "a large quantity of spice per dish", implying that the prepared food had a strong flavor of spices. Alternately, it could be read as "a wide variety of spices", which could be meant to imply that each dish included many spices.
The first meaning - "a large quantity of spice" - usually appears in connection with the mistaken belief that medieval cooks used spices to cover the flavor of spoiled meat. I've discussed this myth and its possible origins elsewhere, so I won't go into it here. Suffice to say, if you want to see my head explode, tell someone it's a fact where I can overhear.
Did medieval cooks use large quantities of spices? This is a surprisingly difficult question to answer. The only recipes we have come from the cookbooks of the wealthy, and almost all of those recipes completely lack measured amounts for ingredients, so there is really no way to know if they put in a lot or a little of any given spice. What's more, even if we did have measurements to work with, what would we use as a comparison? To some people anything more than a pinch of salt is too much. To others anything less than drowning in curry is too little.
Assuming they did use large quantities of spices, one possible reason for doing so presents itself: conspicuous consumption. Serving guests a meal obviously made with great amounts of expensive, imported spices shows the host to be wealthy and therefore influential. There is some evidence to support this in medieval accounts of banquets. Still, I sincerely doubt a host would be successful if he gave a banquet where the guests were served unpalatably spiced food, regardless of how expensive it was.
The second meaning - "a wide variety of spices" - is a bit easier to examine. The list of spices used in medieval European cuisine is surprisingly large and diverse, and a given dish may contain a half-dozen different spices or more. However, this doesn't seem very different from many cuisines around the world (e.g. Indian, Mediterranean, Chinese).
If we take the viewpoint that their use of multiple spices in a dish is exceptional, then is there any possible reason for doing so?
Again, conspicuous consumption is a possibility. A mix of spices though can be harder to identify, and it can still be overdone. If a cook has gone to the expense of putting in rare spices, it'd be a shame if no one wanted to eat the final product.
There has been some recent research that demonstrates how certain spices like cinnamon and cloves can inhibit microbial growth, but given the medieval beliefs about health and disease I doubt that this aspect had any bearing on medieval cuisine. Even medieval humoral theories don't seem to have substantially impacted how spices were used.
On the whole, I think the best answer that we can give for this question is: Because they liked the way it tasted.