Employment agreement - employees quitting when they become useful and affordable

Hi, I am Ismail, It is seen that many organisation making an employment agreement with newly joined employee to stay in organization for 5 years or 10 years. My question is it necessary to make any agreement after giving an appointment letter to the employee mentioning the condition and take agreed copy from employee. Actually I think appointment letter is an agreement . so there is no need to make any other agreement except appointment letter.
I know bond labour is prohibited. But some of employees are quitting job frequently . When company occurred some investment for him/her (like giving salary, training ,Conveyance etc) when they are new, they could not affordable for organisation.
When they become affordable for organisation they are switching their job, in that case bond is bond labour permitted. If permitted what should be the format of employment agreement . what should be the compensation for Organisation.
Please clarify......
9th August 2017 From Bangladesh, Dhaka

Dear Ismail,
What you say is correct. The appointment letter itself is contract between employer and employee. However, to know the exact legal version, you may check contract act of your country.
As far as making employee to sign long-term agreement like five or ten years is concerned, it can be deduced that your company is looking for legal solution for the challenges arising out of shortcomings of your organisation's culture. People will quit the job and companies will have to live and grow in spite manpower attrition. At the same time, organisations need to take corrective measures to strengthen the employee retention. In one of my earlier replies, I have given exhaustive list on why employees quit the company. Click the following link to refer it:
https://www.citehr.com/511164-hr-res...ml#post2189679
Employees stay in the company when they have opportunities to grow and there is brand attractiveness. If you wish to discuss with me on this issue, feel free to call me on WA or Google Duo or Skype.
Thanks,
Dinesh Divekar
10th August 2017 From India, Bangalore

Dinesh sir,
Thank you very much for your great suggestion.
10th August 2017 From Bangladesh, Dhaka