Most languages have a class of words that express properties and behave differently from other words. These are called adjectives. In English, for example, words like "red" or "old" or "tall" behave differently from nouns or verbs. For example, you add -s to verbs in the present tense if their subject is 3rd person singular, like "he sings" or "she eats"; but you can't add -s to an adjective, so you say "he is red" rather than *"he reds". You can put "very" before an adjective ("very red"), but not usually before a noun (you can't say *"very food".) Verbs can't be placed between "the" and the noun (unless you add an ending like -ing or -ed), but adjectives can (you can say "the red car", but not "the move car").
It turns out, according to Dixon 2004, that practically every language - perhaps every language - has at least one separate class of words, definable purely on the grounds of their (morphosyntactic) behaviour rather than their meaning, that refer to properties. This class typically includes words expressing size, age, value, and colour, and sometimes more.
But often, a concept expressed using an adjective in one language is expressed only by a verb or a noun in another. For example, in Kwarandzyəy adjectives come between the noun and the plural marker:
ạdṛạ kədda yu
mountain small PL
"little mountains" (hills)
But there is no adjective "happy" in Kwarandzyəy; instead, you use a verb, yəfṛəħ "be happy, rejoice". And to say "the happy people", you say "the people who are happy/have rejoiced":
bạ γ i-ba-yəfṛəħ
person who they-PF-happy
Moreover, though they may always be distinguishable by some test, they usually tend to behave very much like another word class. In fact, Stassen 1997:30 (link goes to 2003) postulates that in every languages adjectives handle predication (saying "X is red", for example) in the same way as either verbs, nouns, or locations. For example, in English or Arabic, adjectives handle predication like nouns (you say "He is tall", just like "He is a footballer"); in Korean or Tamasheq, they do it like verbs; and some languages, like Japanese, have both verb-like and noun-like adjectives.
So clearly people can do without some adjectives, and clearly the behaviour of adjectives tends to be very similar to the behaviour of some other word class. Why not do without them altogether? It would be easy enough to construct a language where no morphological or syntactic tests could distinguish adjectives from verbs, or from nouns. So if practically every language does take the trouble to distinguish them, there must be some pretty powerful cognitive motivation for it - and some pretty powerful historical tendencies acting to separate adjectives from verbs and/or nouns. The question isn't directly relevant to my current work, but it's worth thinking about.