Venice has many places worth visiting.为什么visit要加ing?为什么没有be

Venice has many places worth visiting.为什么visit要加ing?为什么没有be be worth visit,visit为什么不加ing "Visiting a friend"中,visit为什么要加ing呢? 为什么要加这个词?As a grest many people wiill be visiting the countr It's welll worth a visit.为什么不可以是It's very worth visiting. there are many sheep eating grass.为什么加ing不是正在进行时加be的吗? 为什么要加ing many places of interest 为什么interest后面不能加s “visit many places of 那为什么visit的现在分词直接加ing there are many interesting places to visit there.为什么不是visiti 为什么这句话now前面没有be动词,动词也没有加ing