Respected Mufti Saheb
Is there any specific reasons why somebody would pledge allegiance to a sheikh
In the name of Allāh, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-raḥmatullāhi wa-barakātuh.
The concept of Bai’at (Taking allegiance) is explicitly mentioned in the Qura’an and the Ahaadith of the Prophet Sallalalahu Allayhi Wassallam.
My respected and honoured Sheikh, Mufti Ahmed Khanpuri Mudazilluhu once mentioned that there are two different categories of Bai’at (Pledge of allegiance).
1. To express submission to a Muslim Ruler in an Islamic State, who would be known as an Ammerul Momineen. Hence this type of allegiance would become binding and compulsory upon the Muslim inhabitants of that state.
2. To pledge allegiance to a Sheikh e Kaamil  for the purpose of Islaah (Self reformation). This type of allegiance is not mandatory however preferable if one could not reform himself without any assistance from a Sheikh.
My honored Ustaad and Sheikul Hadith, Mufti Ebrahim Desai Muddazillu mentioned the following reason for having a Sheikh;
When a person experiences difficulties in reforming himself and he requires the assistance of an expert to assist him in the reformation process. In order to achieve that, he takes Bai’at as an expression of determination and submission to obey his spiritual mentor. His Mentor then takes it upon himself to guide his Mureed by which in return the Mureed undertakes to obey his command. Therefore through this type of Bai’at, one could achieve maximum benefit and results, by which he could become from amongst the Muttaqeen and pious servants of Allah.
We make Dua Allah Ta‘ala grants you the ability to remain steadfast on his Deen and follow the Sunnah of our Beloved Prophet Sallalahu Allayhi Wassallam. Aameen
And Allah Ta‘ala knows best
Mufti Luqman Hansrot
 A Sheikh who has excelled in Taqwa, Piety and has the expertise to reform others and one is comfortable and able to converse with him freely and his procedures and methodology does not entail any Shirk elements.