Appointment Letter is must to new hiring along with limited period surety bond

Dear all respected seniors,
We are making service agreement / surety bond with new hired employees for limited period such as 2 years or more.
Now my query is, Issuing appointment letter to employee is necessary along with we are making indemnity /surety bond with employees for 2 or more years.
What is the legal aspect for this?
seeking your valuable inputs.
Thanks

In my opinion while hiring a person on probation, making an agreement for continuous work for two years, after probation, can be legally challenged / and not maintainable, and the same is one sided agreement .