You can find your answer by studying the importance and necessity of symbols in different societies.
Symbols have a great power in conveying messages. Holy Shrines, as religious symbols, play an important role in religious life of Shiâas and even non-Shiâa Muslims, more than mosques do.
It is natural that anyone would decorate and honor what they like. Shiâa believe that holy shrines are very important and necessary for their religious lives, thus they decorate and honor them.
Furthermore, we believe that holy shrines are not just religious symbols. We, according to our traditions, believe that prayers and worships are very likely to be answered by God in those places, because our infallible Imams (a) and some of their children who were nearest ones to God after the Prophet Muhammad (s) are buried in these places.