Examples of BHO are: "Yahoo Companion" which you can use if you access them
a lot such as if you use their E-mail service: the "Google Toolbar" which
you can use if you access them as your primary search engine as many do;
"iespell" which can use as a spellchecker for web based document
preparation; plus many more. Some work well, some do not. No, you can
function without them but the only way to judge if they help you enough is
to try them [most are free]. By the way, I have all those mentioned above
and make constant use of the latter two.
Make your own call.