buy bus tickets in advance?

This requires, to do your question justice, a huge answer without knowing, or even with, where, when and how you travel, e.g. is this a spontaneous or a fixed itinerary, how many are 'we', does your money or time matter.

As a generality, extremely few Spaniards, Portuguese or me would book a bus in advance if you are simply talking of city A to city B, as there will be a fair volume of buses each day. There are discounts by purchasing on-line and printing your own ticket with some bus journeys, if your money matters and/or you want peace of mind.
If you have a fixed itinerary you might want to take advantage of sometimes 60% plus discounts on train tickets by booking in advance. If you want peace of mind and ticket in hand and/or no queueing for tickets you purchase on-line and print out. Find one of the many guides on RENFE and on-line purchase with a google.

'It does not appear that you can take a train from Madrid to Granda, is this true?!' - Presumably Granada, there is a Granda.
It was not true until earlier this month, but at the moment and until early May it will be a combination train and bus journey due to network improvements. You certainly will not be able to see your journey or what arrangements for August, this far in advance.

You do not NEED to purchase bus tickets in advance, but I prefer to. I like the peace of mind, knowing that I won't want a bus that's sold out. If that did happen, it's not the end of the world (there will be other busses), but I like a plan and don't like the idea of waiting around for a couple hours for the next option. Other people would find this too limiting and prefer to be more flexible. Consider your travel style and buy ahead or not depending on preference.

The one time I advise 100% to buy ahead is if you need a late bus to get to your hotel in a new city. If the last bus is at 8, and it's sold out, you could be stuck in city A while having reservations in city B.

I buy bus tickets once there but a day or so in advance of a trip so I am not fumbling around getting tickets when the bus is taking off.

We took the train from Granada to Madrid so normally there is train service; if there is construction on the line, they presumably provide a bus work around. You can also book long haul buses for the trip.

I was told by someone who lives in Granada that the train tracks would be under construction for 6 months beginning in March. If that is the case, it will affect travel in August. I am planning to purchase my bus tickets in advance to Granada because I want that peace of mind, but generally, I purchase the bus tickets a day or so before the bus trip.

Hi Karin,

It mainly depends on your traveling style - if you already have a fixed schedule, or if you think you'll have one before departing, you might as well pay for the tickets online, since they usually require you to select a fixed date and changing it can require additional charges.
If you're not sure of your plans, or if you think you'll want to figure it out as it goes, you'll have some more flexibility of movement and you can buy the tickets then and there .
I'm not sure about the train from Madrid to Granada, at any rate there is no AVE (high-speed) train available, but I think there is a bus you can take. Try finding the dates and fares you'd be ok with at https://www.movelia.es/ - even if you're not buying them in advance it can help to know how much you're going to pay for them and how long the journey will take.

Good luck! :)

I buy bus tickets the day before, just so I can plan my travel day - but they're cheap and very rarely sell out, so you don't need to book ahead.

If you train to Granada, the route is pretty circuitous. Many people choose the train just because it's a long trip either way and the train is more comfortable (in the sense of legroom and getting up and walking around).