Hi,

Are you talking about any third party apps or about the timesheet module? Anyways, if you are asking about the purpose of a field in particular model, it can be varry accordingly as per the one who writes the code.

Commonly sequence will be an integer field that is used to set priority or order of the sequence to which the records are displayed. Also, you can check the help given for the field in the code or UI.

See an example in odoo code,

sequence = fields.Integer(
    "Sequence", default=10,
    help="Gives the sequence order when displaying a list of stages.")

Thanks