Question

Why we heat KMnO4 and filter when prepare it

Answer 1

I assume you prepared it by heating, then filtering with a porous filter (not paper). this is because manganese dioxide can precipitate, and you have to filter it out.

Answer 2

Bob Pursley is correct. KMnO4 decomposes to MnO2 when heated and one must filter, preferable through a "fine" or "ultrafine" Selas filter to remove the MnO2. Also, it is worth noting that KMnO4 solution should be standardized and used immediately because precipitation of MnO2 occurs upon standing only a short time.