Bold symbols usually transport a meaning. Horizontal alignment of dot over hat over bold greek letter. These commands format the argument accordingly eg textbf bold text gives bold text.
In a Latex math environment if you simply type textbf nabla a you will get the following result In the above only is printed in bold.
In the default configuration lower-case Greek letters behave differently from upper-case Greek letters the lower-case greek letters are in the maths fonts while the upper-case letters are in the original OT1-encoded text fonts. In a Latex math environment if you simply type textbf nabla a you will get the following result In the above only is printed in bold. The problem with the above two statements is that they are trying to mix Text mode and Math mode in LaTeX. These commands are equally valid within a maths environment to include text.