Why is it advisable to appoint a Project Manager in the early stages of Project Initiation and assign him the Project Charter ?

The earlier the project manager is involved in the project, the better. I don't see that he/she may be assigned the preparation of the project charter unless he/she is a subject matter expert, but getting involved in the process helps clearly understnading the project goals and customer expectations.

To be part of everything from the project initiation till the closing ,  to avoid  losing time misunderstanding and failurefailure

Project Manager is held accountable to success or failure of the project in-spite of many stakeholders involved in the project.

Hence it is always advisable to appoint a Project Manager in the early stages of Project Initiation and assign him the Project Charter though not necessary. It may be the case where Project Manager comes into picture post handover of Project Charter from Business / Sponsor.However in such cases PM has to all the planning and create scope from the scratch.