Swami Sivananda in "All About Hinduism" Page 115-119 writes
Idol or Murthy (Vigraha), Sun, Fire, Water, Ganga, Saligram, Linga are
all symbols or Prateekas of God which help the aspirants to attain
one-pointedness of mind and purity of heart. A symbol is absolutely
indispensable for fixing the mind. For a Bhakta or a sage, there is
no such thing ...