The Miles + Cash option is definitely a combination of Miles + Cash, but it is only offered in fixed increments (unlike Pay with Miles which is variable in 5K increments). If you select the Miles only option for your flights, it might be something like 480K miles, rather than the 450K miles you are seeing for the Miles + Cash option (what flights and date are you looking at?). So if you don't have the needed 480K miles in your wallet for the award, but you have just over a 450K balance, you can make up the difference by paying cash for 30K of the miles at a rate of about 1.8 to 1.9 center per mile which works out to be a little under $600. If you used to the Delta Buy Miles option to simply purchase 30K miles, it would be somewhat over $1100 to purchase the same 30K miles (3.5 cents per miles + 7.5% US excise tax). So there is limited value to Miles + Cash in certain cases, but it's probably best to look for awards where you already have enough miles to cover the cost of the Miles only option. As noted above, Miles + Cash is only offered on 100% Delta operated awards and there may be cheaper awards available on international awards which include partner operated flights (where your 408K miles is enough to cover entire award). You will need to check the Miles only option to see if there are any of these cheaper partner awards available. If your awards are for entirely domestic flights, then you won't have any partner options available.

Note that if you had 470K miles in your account in the above scenario, it would probably make more sense to simply buy the extra 10K miles with Buy Miles at 3.5 cent per miles as the additional expense would be $376 which is considerably less than the nearly $600 additional you would be paying with Miles + Cash. You'd still have 20K miles left in your account if you used the Miles + Cash option, but if you don't really have any future travel in mind with Delta, those miles probably don't mean much.