1. To support the main part of the instrument
2. To attach the theodolite to the tripod

Pick - out the essential part of transit that has above two distinctive functions.

Explanation:

Components of transit theodolite

Levelling Head

• It is the lowermost part of a theodolite. It consists of two parallel horizontal plates separated by three leveling screws. 
• The lower plate with a large threaded hole in its center is called a trivet or footplate.
• It provides a means to place the instrument on (a tripod) stand and get it screwed.
• Its central aperture provides a way for suspending a plumb bob.
• The upper plate of the leveling head is called the tribrach. It contains a tapered bearing at the center.
• It has three arms each carrying a leveling screw.
• It provides support for the upper part of the instrument.
• The principal use of leveling head is to provide a means for leveling the instrument.​

Additional Information

Plate Levels

• A pair of level tubes are placed at right angles on the upper plate.
• These are used to make the vertical axis of the instrument truly vertical i.e., for leveling of the instrument.

Lower Plate

• It is a horizontal circular plate monolithically constructed with an outer spindle. A scale is engraved at its beveled edge with divisions in degrees and minutes increasing in a clockwise direction.
• It provides the main scale reading of a horizontal angle and a means to fix/unfix the whole of the instrument.

Standard (or A-Frame)

• Two standards resembling the letter A are attached to the upper plate. These provide the bearings of the pivots of the telescope allowing it to rotate on its trunion axis in the vertical plane. The vernier frame and arm of the vertical circle clamp are also attached to it.