F1 involves a very large squad of specialist staff, who would need to be set up in the same way as a football team (perhaps more so, as there are more people). Mercedes started researching for 2014 (the current regulatory phase) in early 2012, so if McLaren holds hope for a similar advantage in the 2021 regulations, it must start doing so in the next few months. Even if the regulations don't look close to settled at that point. Note that McLaren is a relatively well-heeled team, at least for now.
Within a particular regulation phase, improvement tends to be iterative (in that good cars have tended to be a refinement of an almost-as-good preceding car, with the difference being unlocked by money - only 3 teams spent less than $100 m for each second of improvement they got this year), so getting 2021 right is key if McLaren has hopes for a better 2020s than its 2010s are being.
The forecast is necessary - not for us, who can guess what the plan really needs to look like, but for potential sponsors to get that "$100 m per second of improvement" paid for.