what are some reasons?
i only knew the number because of capfriendly!
Cap space is calculated daily, not yearly.
The $83,500,000 cap number is for the entire season, which is 181 days. That means each team has a limit of $461,325.97 in cap space every day (83.5 million divided by 181 days). Player salaries are also calculated daily - Jack Hughes has an average annual value of $8,000,000 meaning that every day he accounts for $44,198.90 against the cap (8 million divided by 181 days).
Every day that we operate under the cap, we save that much cap space for use later. If our daily salary is $452,058.26 (which it currently is according to Puckpedia) against a daily cap of $461,325.97, that means we "bank" $9,267.71 in space for that day. We can use that cap space later to call up players or add guys at the deadline. If the salary numbers are the same tomorrow, we bank another + $9,267.71, which gives us a total of $18,535.42 banked over the season.
Long Term Injured Reserve works a little different - when a player is on LTIR, their daily cap hit still goes against the cap, but we are allowed to exceed the cap by roughly that amount. So if Jack Hughes went on LTIR, his daily $44,198.90 would still be on the books, but we would be able to exceed the cap by a certain amount determined by his salary - this is called the "LTIR pool".
The cap pool is calculated by subtracting the unused daily cap space we have the day he goes on LTIR (in this case, $9,267.71) from the cap hit of the player on LTIR ($44,198.90). This would give us an LTIR pool of $34,931.19 per day. That is the amount we can exceed the cap by every day that Hughes remains on LTIR. The "true" cap is still $461,325.97 a day, but for as long as Hughes is on LTIR we would be able to operate with a daily cap number of up to $496,257.16 a day (daily cap ceiling + LTIR pool). This gives us room to call up players, make a trade, or pick guys up off waivers and have a full roster while still being cap compliant.
So, LTIR allows you exceed the daily cap while still being able to operate. The problem is that if you are over the true cap due to LTIR, you cannot bank daily cap space. So if Hughes is on LTIR and we have a daily cap hit of $475,000, we are cap compliant due to LTIR. But we are still technically over the cap, in this case by $21,257.16. So we won't bank any cap space that day.
So, injuries can really mess up the cap. We have seen this a lot the past few years with Tampa and Vegas having outrageous LTIR pools of like +$20,000,000 over the cap. Teams carrying nearly 100 million in cap but still technically being compliant due to LTIR. Also the use of LTIR to stash expensive players (like Nikita Kucherov) until the playoffs, when the salary cap disappears and they mysteriously become healthy enough to activate. For those teams, they need to be extremely careful about how they trade for guys at the deadline since they are already exceeding the cap - it usually has to be a dollar out for every dollar in.
This is also part of the reason budget teams like taking on dead contracts. While there is a maximum upper limit on how much cap teams can have ($83,500,000), there is also a minimum amount of cap they must spend as well, called the lower limit (which is $61,700,000 this year). A team like Arizona doesn't want to spend that much money since they are broke and not trying to compete this year, so they will trade for contracts of permanently LTIR or retired players. Between Shea Weber, Jakub Voracek, and Bryan Little, Arizona has over $21,000,000 in cap space for players who will never see this ice again. If those guys are on LTIR an unable to play, their salary is usually paid by insurance. Retired players don't get paid at all. So Arizona is 1/3 of their way to being cap compliant while spending little to no actual cash. Both are very good for them, now they only need to spend about $40,000,000 in actual salary on their NHL roster to be cap compliant. It lets them field a team that's cheaper and worse than other NHL teams, which is exactly what they want.
Navigating the cap is extremely tricky and there are a lot of weird situational rules - this isn't even covering things like performance bonuses.
tl;dr - Using LTIR to gain cap relief messes with your ability to gain cap space, which will hurt our ability to add salary at the deadline.