If 冬 in itself means winter, why add 天?
In short, for disambiguation, at least oral disambiguation.

Most of the words in Old Chinese are monosyllabic words. However, when it comes to oral speech, it is too ambiguous to use. As time went by, we developed Modern Chinese, which encourages the usage of disyllabic words, for oral disambiguation.
In oral speech, the monosyllabic word &quot;dōng&quot; can mean 冬 (winter), 东 (east), 鸫 (thrush), 氡 (radon), etc, so it is too ambiguous to pronounce &quot;dōng&quot; only. Therefore, in Modern Chinese, we use the disyllabic word &quot;dōng tiān&quot;, written as 冬天, literally &quot;winter days&quot;, for disambiguation. You can also use the disyllabic word &quot;dōng jì&quot;, written as 冬季, literally &quot;winter season&quot;, for oral disambiguation.
Similarly, there is a monosyllabic word 锋 (fēng) and a monosyllabic word 利 (lì) in Old Chinese. Both of them means &quot;sharp&quot;. However, in oral speech, &quot;fēng&quot; can mean 锋 (sharp), 风 (wind), 疯 (mad, crazy), etc, and &quot;lì&quot; can mean 利 (sharp, benefit), 力 (force), 粒 (grain), etc, so it is too ambiguous to pronounce either &quot;fēng&quot; or &quot;lì&quot;. Therefore, in Modern Chinese, we use the disyllabic word 锋利 (fēng lì) for &quot;sharp&quot;, for oral disambiguation.

Can I just write monosyllabic words but speak disyllabic words? No. They look too archaic. Nowadays, native Chinese speakers are familiar with Modern Chinese, but unfamiliar with Old Chinese. If you write monosyllabic words, native Chinese speakers may feel like &quot;have you just time-travelled from thousands of years ago?&quot; Actually, it is almost similar to the feeling if you speak Classical Latin in Europe.