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Comfortable :- It is comfortable to swallow something that is round shaped rather then tablets that are sharp edged or razor edged. Circular edges have no specific shape and are easy to consume.

Contact with organs:- The circumference of a Circular tablet is much more larger than other sizes. This means it will get in touch with organs, body parts etc easily.

May be it for the purpose of cost effectiveness. As number of tablets can be produced using the same dye and therefore cost can be minimize.
If company has to produce different shape. It will cost more to the company and company has to procure different die for each type of tablet.
On the part of user, it may be easy to take tablets with round shape. .