Q.Why should spotting of mixture be done above the level of mobile phase ?

Chromatography (from the Greek chroma, colour) separates the components of a mixture and purifies compounds, discovered in 1903 by Tswett for separating coloured plant pigments. It works, like solvent extraction, by the distribution of solutes between two phases — here called the stationary phase (fixed in place) and the mobile phase (a solvent or solvent mixture that moves over/through the stationary phase, carrying the loaded mixture with it). Depending on their relative affinity for each phase, components either linger on the stationary phase or move with the mobile phase, gradually separating.

When the stationary phase is a solid, it is called Adsorption Chromatography, based on Differential Adsorption (different solutes are held back to different extents). It has two forms: Column Chromatography, where a slurry of alumina or silica gel is packed into a stopcocked glass column, the mixture loaded on top, and the mobile phase poured through — solutes separate into visible bands, the most weakly adsorbed eluting (leaving) first and the most strongly adsorbed last; and Thin Layer Chromatography (TLC), where a $0.2\,mm$ layer of silica gel/alumina on a glass chromplate is spotted with sample about $2\,cm$ from one end and developed upright in a closed jar of mobile phase (spot kept above the liquid), the components separating into spots at different heights. …