It appears to be an attempt to make it easier to find the beginning of the next line after you finish reading a line. However, the effect is much too subtle to be of use, and even if made more prominent, it would still ruin the typographic color and generally confuse readers.

Better solutions include:

Using a more optimal column width. 50-60 characters per line is a commonly suggested range for most media.

Using a more optimal leading (line-height). "Set solid" type (ex. 12px/12px) is almost always a bad idea, particularly so in a long block of text. Depending on the type, media, etc, ratios in the range of 1.2-1.8 are a good starting point for improving readability.