Robert Koch assistant advised him to use agar instead of gelatine for solidifying culture media for cultivation of bacteria as -

Agar is preferred to gelatin as a solidifying agent in culture media. This is because gelatin melts at relatively low temperature (<35o C), while agar melts at around 85° C. Therefore, when gelatin is incubated at the normal temperature for bacterial growth, i e. at 37° C, it melts, while agar remains solid.