noun, suffix
Used as a suffix or noun to indicate a physical side, a party in a dispute, or a particular aspect. The reading がわ is standard; かわ is a rare variant.
道の反対側にコンビニがある。
There is a convenience store on the opposite side of the road.
彼はいつも弱い者の側に立つ。
He always takes the side of the weak.
noun, suffix
Specific use for the casing of a watch or clock. Not a general word for 'case' or 'box'.
この時計の側は18金でできている。
The case of this watch is made of 18-karat gold.
Kana spelling is common in everyday writing, especially when the kanji is not needed.
The reading がわ is a voiced form of かわ (side), which is related to 方 (かた, direction/person). The kanji 側 is conventionally used for this meaning.