I often have people in my lab come to me with requests for non-day-to-day things they would like to purchase for our research. They come to me with travel requests or questions about committing funds to something next summer. They are planning ahead and that's a good thing. But I can't always answer them right away.
Why? Because budgeting for a lab is a balancing act with many unknowns.
A lab budget is a bit of a puzzle. At the moment the lab runs off of five different pots of money that vary significantly in the amount and duration. Some are scheduled to end this year, some a few years from now. Some have more or less restrictions with what you can do with the money. Each one of those pots has sub-categories with different strings attached. Whereas you can rebudget between most of the categories, they don't all work the same way (e.g. Equipment).
The single biggest charge to any grant is personnel. In a lab environment, the goal is personnel turn-over. Of course, when people leave and when new ones arrive is a moving target. A PhD student might leave after three years or take over 5 and there is little way to know a couple years in advance which will happen. Spending heavy now could mean leaving someone out in the cold in two years.
All of the different pots of money can interact. People often work in overlapping budgetary areas. Reagents exist within the intersections of the lab budget Venn circles. Services? Software? Consumables? There's no way to charge everything in a project-specific way*.
Given the constantly moving parts, in particular the people, a lot of budgeting just can't be done on the fly. If we spend $20k from Pot A, how is that going to affect the activities supported by Pot B? If everyone goes to Big Fancy International Conference, will we have the $$$ to pay everyone the following summer and still fulfill our research goals? Will Sue still be here and in need of support? Can we pick up Bob on an RA for another semester so he can finish after has last experiment imploded?
Ideally, I'll have made the right decisions to ensure that we keep people paid and keep doing good science, but to make that happen each significant expenditure needs to be thought out.
*We do the best we can. Please don't audit me.