The selling club wants 50m.
The buying club isnt willing to pay 50m.
So to appease both clubs they stick on add ons to make it to 50m.
The selling club is happy cos if the player is a success they get their 50m.
The buying club is happy cos if the player FLOPS, they only pay 35m.
Honestly ive had this attitude for a long time, people just really want to ignore add ons, now whilst some add ons will never be achieved, IMO most are at least targets that clubs think can be reached.
I mean take Martials transfer to United :
The document says the initial fee of 50m Euros (£38.5m) for the player is to be paid in two instalments - €20m (£15.4m) within three days of his transfer, and €30m (£23.1m) on July 1.
There are then three further potential payments - €10m (£7.7m) if the player scores 25 first-team goals for Manchester United, €10m (£7.7m) if Martial makes 25 appearances for the French national team and €10m (£7.7m) more if the player is ever nominated for the Ballon d'Or award.
There is also a sell-on clause inserted in the deal, with Monaco due to be paid 50 per cent of any transfer fee between €60m and €100m United receive for the player.
2 very achieveable add ons and 1 stab in the dark.