whistleThe variety of equipment referees can make use of has become larger and larger over the last couple of years and actually even decades. While things like headsets, spray, goalline technology and even cards have been comparably innovative tools in the past, all started with a whistle. But blowing the whistle is not needed in every match situation as the Laws of the Game define. Moreover, the way you whistle can have a significant impact on the effect on the players (often called "whistle language").

 Please watch the video:



The first and second clip show violations of Law 5 and its guidelines (FIFA LotG p. 83). Both referees should be reminded on situations where the whistle must be used mandatorily.
The whistle must be used in these cases:
1) To start play (1st / 2nd hald), after a goal
2) To stop play
    - for a free kick or penalty kick
    - if the match is suspended or abandoned
    - when a period of play has ended due to the expiration of time
3) To restart play for:
    - free kicks when the appropiate distance is required
    - penalty kicks
4) Restart play after is has been stopped due to:
    - the issue of a yellow or red card for misconduct
    - injury
    - substitution
The whistle is NOT needed in these cases:
1) To stop play
    - for a goal kick, corner kick or throw-in
    - a goal
2) To restart play from
    - a free kick, goal kick, corner kick, throw-in
Even though the Laws of the Game do not define it, it is also recommended to use the whistle after public, verbal warnings against players. This emphasizes the importance and efficacy of your verbal warning.
Furthermore, the Laws of the Game recommend to deploy the whistle carefully as it otherwise might lose its efficacy and that the referee should proceed pro-actively in situations where the whistle is needed: 
"A whistle which is used too frequently unnecessarily will have less impact when it is needed. When a discretionary whistle is needed to start play, the referee should clearly announce to the players that the restart may not occur until after that signal."
In the first two video clips, you can notice the players not being reminded on the need to wait for the referee's whistle. Instead of blocking a quick free kick, the referees fully tolerate it and even continue to write down the player's number into their book (2nd video) and putting the book back in their pocket while play has already started to flow. The 2nd video is even more striking as a promising attack is ongoing while the referee is still busy with his book (and this means that he cannot be concentrated by 100% as he wastes some cognitive, attention-linked resources). You can even notice his assistant referee not being ready either - he is not in line with the 2nd last defender at a set piece (00:42 in the video), which could have created trouble in case of an offside situation. 
Damir Skomina had solved that much better earlier in the same match (3rd clip). Also here, the Dutch team were about to start a quick free kick enabling them to take an advantageous position. However, Skomina had just issued a yellow card and had not even started to write down the number of the player. He correctly ordered the free kick back and communicated this procedure with adequate body language gaining acceptance for it.
So: Never allow players to restart play after a disciplinary sanction or other match situations listed above as long as you have not used your whistle yet. A whistle is necessary and mandatory in some cases and most referee observers deem it as a clear point to improve and light form of violating the Laws of the Game if you fail to be aware of that.