I can't see how orphans and widows can ever be a "good thing". They should in my opinion always be avoided, but in some cases you can be forced to accept them because the alternative is worse.
Why are orphans and widows a bad thing?
It always hurts my eyes to see those tiny snippets of text. It looks like an unintentional error. In this example we have both:
The orphan makes the readers start reading a sentence and then immediately forces them to move their eyes to the top of the next column or page or to turn the page. If the paragraphs are indented it also cuts off the corner of the text block in the lower left corner, making the pages look unbalanced.
The widow can also feel annoying for the readers since they have to remember the start of a sentence then find the continuation and might be disappointed to find just a tiny snippet. The widow can also make the page look unbalanced since it often cuts off the upper right corner of the text block. Another bad thing about widows is that they in some cases can look like a heading.
How can they be avoided?
(I'll assume InDesign is used, but similar approaches can probably be used in other applications.)
Initially you can set the Keep Options of your Paragraph Style to disallow orphans and widows. Check Keep Lines Together. Choose At Start/End of Paragraph and set Start and End to (at least) 2 lines:
This doesn't really fix the problem. It just moves the troubled line introducing an empty line instead. In some layouts this can be good enough, but in any case it makes it easier to spot orphans and widows.
When all text has been styled you have to go through your text, spot the orphans and widows and try to fix them manually.
In simple one-off layouts, the first thing to try is to change the design until the problem goes away. Change the font size, margins, number of columns etc.
In dynamic magazine layouts you can try to organize your elements differently. Reposition text frames and images, let headings span over a different number of columns, use text wrap etc.
If all else fails, and in long plain texts with no images like a novel, you have to manually adjust paragraphs to become one line shorter or longer. You have two knobs to turn:
Tracking. Increase or decrease the spacing between letters. Use a tiny amount of positive or negative tracking (probably not more than +-10).
Word spacing. Increase the spacing between words with Ctrl / Cmd + Alt + \ and decrease with Ctrl / Cmd + Alt + Backspace.
The two methods can be combined to achieve the best (or least bad) result.
Sometimes you have to go back several pages to find a paragraph which will change its length with only a small amount of adjustment. And sometimes you need to change several paragraphs before the puzzle is solved.
Be aware that there are cases which logically can't be solved no matter what you do. In those cases you'll either have to live with an orphan (widows are worse in my opinion), try to convince the writer to change the wording or simply "cheat" by having a spread with one line less on both pages. Maybe nobody will notice.