Boundary Extractor was introduced with JMeter 4.0. It allows to do correlation pretty much the same way as web_reg_save_param in LoadRunner. With earlier versions of JMeter one had to use regular expressions for correlation. With Boundary Extractor one can specify left and right boundaries to capture the dynamic string.
Addition of Boundary Extractor is one of the best developments in JMeter 4.0, it would make it very easy for those who switch from LoadRunner to JMeter and vice-versa. The LR to JM utility from PerfCritic makes use of Boundary Extractor when converting LoadRunner scripts to that of JMeter.
Boundary Extractor allows to use web_reg_save_param expression from LoadRunner as it is. If you were moving from LoadRunner to JMeter, in earlier versions of JMeter one had to use regular expressions and convert web_reg_save_param expression to a regular expression. Similarly if you were converting your JMeter script to that of a LoadRunner script you had to convert your regular expression into a left boundary right boundary web_reg_save_param expression. Boundary Extractor of JMeter 4.0 fills this gap that allows easy conversion between two formats. Also Boundary Extractor makes correlation easy in JMeter when compared to writing complex regular expressions.