Extraction of mandibular first molar in an 8 year old child is known as:
Wilkinson Extraction 
Wilkinson advocated extraction of all the four first permanent molars between the age of 8 ½ & 9 years. 
The basis of it is the fact that first molars are highly susceptible to caries. 
The other benefits of extracting first molars at an early age are:

To avoid third molar impactions by providing additional space for their eruption.  
To reduce crowding in the arch.
Wilkinson's extraction
Compensatory extraction
Serial extraction
Orthodontic extraction